First off, I would actually know the answer to this question because my coach is a Grand Master (Joel Benjamin). The first thing he told me was that chess is 99% tactics. This is really true. This advice may sound ridiculous, but try playing some gambit openings like the danish gambit, fried liver attack, and other ridiculous attacks. These attacks usually work, and I am very strong with them. These attacks arent "undefeatable" but they teach you tactics, because one tactical mistake in an attack will lose. Next, if you lose a game, look at your game. If you play online, look at the notation and see what you did, or where you went wrong. This definitely will help you. After a while of tactics practicing and other stuff like that, I reccomend finding out what the major weaknesses in your game are. If you are a poor positional player, then work on your positional play. Usually you will need a coach for this to improve your theory in positional play. You can usually get a book on positional play for this as well. Once you are done with the opening and middlegame, I reccomend that you look at your endgame. You have to know when a position is a draw, and how to keep it that way. If it is a win, play for the win then. Now, I will go into more depth on each section of the game:
Figure out your style. You need to know what type of player you are, and choose an opening that fits your style. DO NOT CHOOSE ANOTHER OPENING UNTIL YOU HAVE MASTERED THE ONE YOU ARE CURRENTLY PLAYING. I reccomend that if you are just beginning chess, get used to e4 e5. These are open games and lead to the most tactical play. I also reccomend from white the Italian Game, which is very fun and tactical to play. As you get more advanced, try to learn the sicilian defense. I personally like the dragon variation.
Middlegame: Tactics, Tactics, Tactics. The one who knows tactics better will win. You need to know tactics and pawn structural ideas. Thats about it. If you choose an attacking opening, then you will want to attack in the middlegame. If you choose a slow, positional opening, you will want to play a slow, positional game.
endgame: Think before you move, and try to make past pawns. Past Pawns win games, and protected past pawns shut your opponent down. Do your best with that. I coach as well if you ever need any help. I play on both ICC and chess.com. My user on chess.com and ICC is pawnpusher3.
I hope I helped you. One last thing, when analyzing, also look for the better moves, not just why they are wrong. When you find bettter moves, this will improve your board vision, which really improves your whole game overall. Also, I reccomend investing in a chess engine, such as Fritz or Rybka, because these really help. I personally use Fritz, but I hear Rybka is very nice as well.
What I learned from him
My rating is 2000 in Real life (USCF)