Here are about 50 previous threads on the subject. Bound to be some good advice in there somewhere...
http://www.chess.com/forum/search?keyword=how+to+improve
And of course if you're really serious, here's a link to about 10 years worth of monthly articles by Coach Heisman. Enjoy!
http://danheisman.home.comcast.net/~danheisman/Articles/Novice_Nook_Links.htm
lately I've thinking about it, I've done some things like: tactics, middle game, a little bit of openings.... However, I would like to know more!