In Magic, it’s really important to always have a plan for how you are going to win. In particular, one player should always be trying to bring the game to a close, while the other should be trying to draw it out. NB though that roles can sometimes switch mid-game, or in between games as the result of sideboarding. “Misassignment of Role = Game Loss.”
Who’s the Beatdown - Michael J. Flores

You typically win a game of Magic by accumulating small advantages in terms of cards, resources, or time over the course of the game. Most strategies will tend to focus on one or the other, but you can trade resources for time and vice-versa.
Tempo and Card Advantage. - Eric “Danger” Taylor

In a similar way, you can exchange cards for life, and vice-versa.
The Philosophy of Fire - Michael J

“Investment” is simply the resource/effort cost that a card requires in order to get the payoff: “In general, I would say that for you to play with cards that require investment, you need to have some way to get long-term card advantage with it. Examples of these cards are Disrupting Scepter and Goblin Bombardment. On the other hand, cards that generate short-term utility but long-term card disadvantage, such as Mana Severance, should be avoided in non-combo decks.”
Investment - Michael J

The “Fundamental Turn” is the turn on which your deck is trying to win. Ideally, you either want to be one turn faster or two turns slower (and correspondingly bigger + more disruptive) than your opponent.
Clear the Land and the Fundamental Turn - Zvi Mowshowitz

“Definition: A player is said to have inevitability if and only if from the current position he will win a long game. A player is said to have inevitability in a matchup if and only if they have inevitability on turn one… In a large percentage of Magic games and matchups, the goal of one deck is to have inevitability and then preserve it long enough to take advantage of it. Preserving inevitability sometimes just means staying alive, but it can mean other things as well. Often it means not losing too much card economy along the way. Every traditional control deck works like this.”
Who’s the Beatdown II: Multitasking - Zvi

You can beat a lot of people if you can avoid beating yourself.
Stuck in the Middle With Bruce - John Rizzo

Some great nuts-and-bolts articles by Frank Karsten:
10 Useful Interactions Every Modern Player Should Know
10 More Useful Interactions Every Modern Player Should Know
How Many Colored Mana Sources Do You Need to Consistently Cast Your Spells?

Other resources:

Limited Resources Podcast
Playing to Win
Simulate all kinds of stuff with the Stat Trek Hypergeometric Calculator!
EDHREC is a great starting point for building commander decks.
17Lands

There are some great insights in the articles linked above, but at the end of the day if you want to get really good at anything, you have to practice a lot. Prep for Constructed by playing your deck, a lot, against the decks you expect to see. Ideally, a few turns into the first game you should have a pretty good idea of what’s in your opponent’s deck, how their specific cards match up with yours, and your general plan for the game. You should definitely know very quickly whether you are the beatdown or control player; remember that misassignment of role = game loss.