Magic: The Gathering can be played on many levels.
Winning at Magic is about making decisions to maximize your chances of removing your opponent's option to continue to play the game.
There are three basic types of resources in Magic. They have no inherent value beyond your ability to leverage them to give you more or better options or deny your opponent the same. When one player wins, it does not matter how few resources they had or how many resources their opponent had. These resources are delineated by how you acquire them.
1. Life (often described by something called "The Philosophy of Fire")
2. Tempo (rarely spoken about in polite company, but important to understand)
3. Card Economy (also commonly referred to as card advantage)
You begin the game with twenty life. The first nineteen have no intrinsic value, which is to say, they do not inherently give you options. The last one gives you the option to continue to play the game. Removing your opponent's last point of life makes them lose.
While the most commonly relevant resource of this type is life, it also includes any other resource you begin the game with and do not accrue more of naturally. For instance, you begin the game ten poison counters away from losing. Like life, only the last one (the tenth poison counter) has inherent value.
You begin the game with 60 cards in your deck, and trying to draw a card with none remaining makes you lose the game. You may put more than 60 in your deck; however, this is generally inadvisable because the marginal utility of being one card harder to deck is worth less than the decrease in probability of drawing your 60 best cards that comes with each card added above 60. Some formats use smaller decks (like 40 in Draft), others larger (like 100 in Commander), but the principles remain the same.
While most resources of this variety keep you from losing, there are a few niche cases involving resources that also fall under this umbrella. For instance, everyone begins with access to a fifteen-card sideboard. If you have cards that require more cards in your sideboard to use (such as Wishes), you may have less of this finite resource to work with, and over the course of the game, it may decline further.
Tempo resources are ones you do not begin with but naturally acquire over time (typically per turn). These include the untap step, the land drop, and the attack step, among others. These resources are only temporary, and if they are not used, they evaporate.
The most commonly discussed tempo resource is mana. You begin the game with no mana, but you acquire it from tapping lands. You also begin with no lands; however, you can play one a turn. If you don't use your land drop for the turn, you don't get anything. The opportunity just goes away. Still, this doesn't mean you should always utilize this option. Cards on the battlefield are face up nearly all the time and known to both players. Cards in your hand are known only to you, which is an advantage.
Once you play a land, you can tap it for mana each turn. If you don't use the mana, you lose it. It is like you spent it anyway; you just don't have anything to show for it. As a result, playing a mix of cards that increases your probability of spending all of your mana every turn is generally advantageous.
Likewise, creatures can attack every turn, so not attacking with a creature sacrifices tempo. However, in exchange, you gain the option to potentially block with the creature. Opportunity cost is at the heart of Magic strategy. When you are choosing between options, it's important to consider what you're giving up, including the other options you could have taken.
"Cards" are resources you begin the game with and accrue more of naturally over time. In the most basic form, these are the seven cards you begin the game with, as well as the card you draw each turn.
Since cards are "options," it is generally better to have more cards than your opponent. This is said to be card advantage. The quality of options matters, too. The best card in your deck is worth more than a random card from your deck. High-casting-cost cards generally do not give you options before you can cast them; however, once you can, they generally afford you more powerful options than cheaper cards.
The 60 cards in your starting deck are not the only source of card economy. Some cards create tokens. While a card that generates multiple tokens doesn't draw any cards, it is said to generate virtual card advantage, since casting Dragon Fodder is largely the same as casting two Mons's Goblin Raiders. Again, quality of options matters.
Some cards work from zones besides the battlefield. These cards either function like spells that can be used once (with mechanics such as flashback, embalm, eternalize, unearth, scavenge, evoke, haunt, rebound, cycling, and reinforce) or permanents that can be used repeatedly (with mechanics such as forecast, cipher, madness, buyback, recover, retrace, or unique abilities such as that on Firemane Angel). They are all just options, and whether to consider them "cards" or not is a function of how relevant their options are.
A Firemane Angel in your graveyard is like a difficult-to-remove permanent that gains you 1 life per turn. A Champion of Wits in your graveyard is like a 4UU spell that casts a 4/4 that draws four cards and then discards two when it enters the battlefield.
Over the course of the game, you are confronted by options to trade resources for other resources. If your Elvish Mystic taps for mana, you gain some tempo; however, you may be sacrificing a point of damage from not attacking.
If you block a Craw Wurm with the Elvish Mystic, you are trading it for six life points. If you are at 25 life, you could just make that trade next turn, but may also have the option of double-blocking if you draw a creature. By contrast, if you're at six life, not blocking may lose all of your other options. This is where the real value of life comes in. Every life point increases your options with regards to which and how many creatures not to block. Additionally, cards like Wooded Foothills cost life to use, so the difference between being at one life or two can be the difference between having enough land and being one short.
Learning the optimal sequencing of lands is a deep subject and is generally best done with focused practice and analysis of what worked and how things would have worked out if things had played out in a different order. The basic strategy, however, is to spend your mana as efficiently as possible. If you have a chance to play a tapped land on a turn you would have otherwise wasted a mana, you typically want to do that.
You want to play out your lands in an order that first and foremost enables the cards you have in your hand. Secondarily, you'll want to enable the other cards in your deck, so you have the option of playing them when you draw them. For instance, if you know you have four Cryptic Commands in your deck, you usually don't want to play basic Mountain and basic Swamp in the first four turns (unless you have cards like Sunken Ruin).
There are no end of tactical considerations that may cause you to do things abnormally, but it's still important to know the basic strategy. Strategy is your overall plan, your general big-picture approach, while tactics are the means used to gain a specific objective.
When you play a sorcery or instant, you are spending both mana (tempo) and a card. In exchange, you are impacting you or your opponent's future options by whatever the card says. Whether to put the card into your deck or not is one question; however, once you are in a game, the real option is between whether to pay the mana and get the effect now or to save the option for later (but not get the benefits of the card yet). The "card" is already spoken for (with plenty of exceptions, such as cycling, which is really just the option to trade this card's options for another card's), though the information as to what the card is may be of value.
Noncreature permanents give you one or more options in exchange for resources (whether mana, cards, untap steps, or whatever). In addition to costing you the mana (and the card you committed by putting it into your deck), they cost you information. When you have a card in your hand, you know something they don't know. When you put it on the table, you have ceded that advantage as part of the deal of getting to use the card.
When you play a creature, you are trading mana and a card for a permanent with the option of blocking each turn, that on your next turn gains the option of attacking or blocking.
When you attack, you are risking the creature(s) for the prospect of dealing damage to the opposing player. Each of your creatures has this option, and generally not until after the opposing player has had a turn to prepare for them. On balance, you generally know what options your opponent has to block with before committing to the attack.
Some cards have haste or "virtual haste," which is one of the best abilities not only because of the extra damage the card can deal but because of how much less information your opponent has to work with. For instance, an opponent may need to keep a blocker on defense in case you cast a haste creature, missing out on damage. Then, on your turn, you could just cast something else and save the haste creature for another turn.
When you attack, you are also implicitly offering deals to your opponent, with the threat of damage if they do not accept. If you attack with a 2/2 and they have a slightly better 2/2, they may not want to block. However, if the two life is worth more than the relative increase in card quality, they may choose to block anyway. This can be particularly true when there is a good chance that whatever you do after your attack phase is likely to make their 2/2 unable to attack.
When you block, you decide which deals you are accepting. These deals are of much greater variety than just two creatures trading off for each other. For instance, if they attack with three 2/2s and you have a 3/3, they are offering you one of their 2/2s in exchange for four life. Since the penalty for not accepting is taking six damage, accepting is usually a pretty clear choice. However, creature combat carries with it some amount of risk. They could have a pump spell or some other way to slant the deal in their favor.
When someone attacks you, it's important to remember that they knew what you had available to block with. Look at what deal they are offering you and ask yourself if that deal makes sense based on what information you have. If it doesn't, what cards might they have in hand that would make the attack make sense?
It's important not to attack and block in constant fear of everything they could have. If you make them use a combat trick, at least they won't have it anymore. If you are going to try to play around it, ask yourself if you're going to play around it next turn too, and so on. Frequently, forcing them to spend their mana playing their trick means less mana available for putting more threats on the table, so the earlier you force them to do so, the more turns you delay other threats.
It is also possible they are bluffing, or have even made a mistake. And it's not about what's a good deal in the abstract. Maybe they generally wouldn't throw away a 2/2 for four damage, but if you're at ten life and they have burn in their deck, it could be a different story. Besides, maybe they believe the 2/2 is unlikely to be worth more than four damage in the future and just want to cash it in.
When deciding how to block, you generally want to figure out what you need to do (if anything) to not die. For instance, if you are at five life and your opponent is attacking with ten power worth of creatures, you've got to block or deal with at least six power worth of attackers or you'll die.
Lining up advantageous blocks has a lot of depth to it, but the basic strategy is to line up blocks efficiently. Barring tricks, a 3/3 is exactly as good as a 4/4 for stopping a 2/2, but the 4/4 is much better at blocking 3/4s. Everything else being equal, you'll generally want to win battles by thin margins. However, you also want to take into consideration what cards you suspect they might have. You'll also want to consider how important any of the individual battles are. You may choose to triple-block a key attacker to make sure it dies, even if it means letting two damage through.
How much to value life points at is another deep and nuanced subject; however, they are generally worth less than most players value them. This isn't to say that you want to just recklessly take damage. However, very often, trading permanent resources for damage just makes your situation worse, since they will be threatening the same thing next turn and you'll have even less to work with. Sometimes there are other factors that will make the next turn different, and sometimes you just can't help it. Generally speaking, though, you want to try to set yourself up for more or better options in the future.
Stages of the Game
Magic is a game with several meaningfully different stages. The basic strategies to employ at each of these stages are different enough that it's important to be able to understand which stage of the game you're in.
At the beginning of the game, you'll have the option to shuffle your hand back into your deck and get a new hand with one less card. You can repeat this as often as you like (though taking mulligans at all should be quite a bit less common that keeping your seven). Then, if you took one or more mulligans, you'll get to scry 1, which helps insure your opener has a playable mix of lands and spells, even if it's a card smaller.
When determining whether to keep your hand or mulligan, it's useful to visualize how the next several turns can play out. This is another deep subject, but the basic idea is to evaluate the range of possibilities, what the chances of each possiblity are, and what each of them means to your chances of winning the game. You don't need to be favored to want to keep a hand. You just need its chances of leading to you winning the game being better than your chances of winning the game after taking a mulligan. What if you draw the land you need next turn? What if it's the turn after that? What else could you draw that you'd be able to play?
The early-game is a period where you and your opponent are both basically manascrewed and unable to play most of your cards. As you play lands and draw more cards, you'll transition into the mid-game, where you can play most of your cards, but not all at the same time. Notice players don't necessarily stay in the same stage of the game as each other. Additionally, it's possible to knock your opponent back down to a lower stage with land destruction or other forms of disruption.
The late-game is where you basically have as much mana as you want and can cast whatever you draw whenever you want. While lands drawn in the mid-game still open up more options, lands drawn during the late-game are frequently dead draws. One of the basic challenges to overcome in a game of Magic is the natural trajectory towards a state of being "out of gas." Once you've used all your spells, drawing an unneeded land isn't just one less threat; it can be an entire missed turn if you have nothing to spend your mana on and no attacks you can make.
While every deck uses their stages of the game differently, at a high level, it's important to determine if you are supposed to be the aggressor (the "beatdown") or if you are supposed to prioritize defending yourself from opposing aggression. This isn't to say that the aggressor shouldn't block, and vice versa. Rather, it is about understanding if you want to stretch the game out or contract it, speeding up the end.
Generally, a player with an advantage wants to get the game over with before their advantage dissipates. Aggressive strategies are typically better able to generate an advantage early on, but as the game drags on, their advantage fades. However, if you are playing an aggressive strategy and have an advantage that is likely to grow over time, you may well decide to slow down and nurture that advantage. Even if you're a 60/40 favorite now, slowing down might make you an 80/20 favorite if you pick the right spot to do so.
Likewise, control decks often start out behind and are built to gain advantage the longer the game goes. As such, they often prioritize "not dying" much higher than more proactive goals. Even still, they must be mindful of when they should go "beatdown." If you're at ten life with a control deck against a burn deck, you easily may want to tap out to play a Gurmag Angler to start attacking with rather than keeping up Counterspell mana. Every turn sooner you end the game is one less chance at a burn spell you have to deal with.
Midrange decks can sort of play aggro in some matchups, or on some turns, while being the "control deck" in other matchups, or at other stages of the game. While fast aggro and pure control can often make the aggressor quite clear, there is a lot of depth of skill to figuring out your role with a midrange deck.
Combo decks defy the usual metrics for measuring advantages. While attacking is the primary way to win, combo decks often aspire to create game states that win by way of some interaction between cards that is greater than the sum of its parts. If a deck contains three cards that create a loop that wins the game, it cares much less about incidental creature damage. Likewise, it may not need to defend itself nearly as much, since one of the best ways to defend yourself is to reduce your opponent's life to zero.
When playing with or against combo decks, it's important to figure out what matters. Ask yourself, "What am I trying to accomplish?" You'll also want to ask yourself, "What are they trying to accomplish?" How can you balance furthering your gameplan with disrupting theirs?
What should the final turn look like? What can you do to defend yourself against that, or ensure your opponent can't stop yours?
What cards do they need to assemble to win? Where are the weaknesses in the chain? Where are the opportunities to potentially do something to interact?
Focus only on what matters.
When you notice your mind flirt with something that doesn't matter in this moment, let it go. Remind yourself to focus only on what matters.
Focus in practice, too. Focused practice is substantially more valuable.
When preparing for an event, make sure you have a clear picture of what the objective is. Then hold your actions accountable. Are they furthering your quest for this objective? What do you need to accomplish to reach your goal? Always aim higher.
Part of preparing is selecting a strategy. Deckbuilding is another rich and complex subject. The short version is to start by gaining familiarity with existing strategies and making sure you understand why they play the cards they do, what they are trying to accomplish. Then you can use them or parts of them as templates for your own decks. How many cards do they use of each cost? How many lands of each color? How many removal spells? It's important to look at a range, with a heavy bias in favor of decks that have succeeded at major events, and doing your homework makes a big difference.
Another part of preparation is practicing play. Tight technical play wins more games of Magic than all other factors combined. When you practice, don't allow yourself takebacks. Make sure to practice sideboarding as well as Game 1s. After all, roughly 60% of your games will be after sideboarding in tournament play.
When you sit down to play a set, it is better to decide upon a number of games beforehand. If you are in the early stages of building a deck, you may need to make serious revisions early on once flaws are revealed. However, when testing "real" decks, play a ten-game set (maybe four pre-sideboard, six post-sideboard), rather than just looking to play long enough to get the result you're looking to see.
Avoiding biases is tricky business, but holding yourself accountable can make a world of difference. After each session, record what you tested, what you learned, what you're still thinking about, what surprised you.
Make sure to surround yourself with like-minded people, even if doing so online is the best avenue for that. The better the people you practice with, the better you'll get. Likewise, the more you help others improve, the stronger the competition they can afford you.
When the event actually comes around, make sure you're doing the intangibles right. People play much better with a good night of sleep. Get in the habit of drinking water regularly, and make sure you have food available so that you're not trying to play hungry. Come early, look around, and get acclimated. When leaving between rounds, set an alarm. Usually, at bigger events, pairings won't go up for several minutes after the scheduled end of the round, but it is much better to be back comfortably with time to spare.
Good sportsmanship really is very beneficial for your improvement as a player for many reasons. Remind yourself of this, rain or shine, win or loss. After your matches, analyze what you could have done differently to give yourself better chances. What would you have done differently if you knew then what you know now? Resist the urge to complain to people about how unlucky you got. Trying to convince yourself that you're already "good enough" may help address the pain you feel, but that pain is the burn of a good workout. Rather than focusing on the parts you couldn't control, look for any possible thing you could. Those are the useful parts.
Do you want to avoid the pain of losing? Don't compete.
If you want to get better, you've got to face your failings. Everyone makes mistakes. Masters see these as opportunities for growth. Instead of telling people about times you got unlucky, tell them about the mistakes you made. When you admit them out loud, you can no longer run from them. That is how you defeat them. Take them. Own them. Internalize what you'll do differently next time.
Be honest with yourself.
Focus on what matters.