Play Slower

There is no way to get better at chess fast. Everything takes practice and patience in order to reach perfection. You must avoid playing blitz or bullet too much, especially when you are in the learning phase. The thrill of playing fast games is tempting but then chess becomes more like a video game rather than something that you want to seriously study and play. Before focusing on opening theory, you should have a good understanding of endgame. Do lots of puzzles, this makes you think more tactically; and if you are young, then you'll pick this up relatively quickly. Playing against strong computer opponents is also a great way to improve your tactics. That doesn't mean you shouldn't play with human opponents. And like others said, you have to pay attention in game and try to analyze as much as possible. Don't just always think two or three moves ahead, try to think deep but at the same time you must try to understand which combinations are worth spending time on (it'll come gradually). In addition, analyze your games after you've played them, both your wins and your losses. See what you could have done better. See what your opponent could have done better. Post game analysis is very important in developing your game. A good way to train yourself to get rid of most of the "blindness" that makes you blunder pieces and fail to see simple tactical combinations is to practice solving tactics puzzles. Play online at sites like freechess.org or chess.com, where you can see if your rating is improving or not. In each of the following worth-solving puzzles, Black is to play and enjoy.