Hi,
Welcome!
Fromper's advice is spot on. Mastering tactics will probably make the most dramatic difference in your game at this point. There are some good sites that offer tactics tutorials and exercises:
Chess Tactics Explained in English: A Complete Tutorial.
Chess Tactics Server
Chess Tactics Trainer
Of course, there are also lots of good books with puzzles such as,
Sharpen Your Tactics.
Also, as Fromper said,
Dan Heisman's columns are excellent. Here's another excellent site that offers all kinds of free resources:
Exeter Chess Club.
Aside from that, play as often as you can, preferably against players who are just a little better than you, so that you are stretched, but still have the possibility of winning if you play your best. Also, I recommend playing turn-based correspondence-style chess or at least really slow games, as opposed to playing blitz, as I believe it's important to take your time and develop the habit of analyzing positions and picking the very best move you can come up with, as opposed to playing quick moves by reflex under constant time pressure.