How To Get Better At Chess

1. Watch better players play, and study games from better players. There are many great games by World Champions and Grandmasters at places like Chessgames.com. It would be best to play through games that are explained (annotated) at a level you can understand, otherwise you won't know why they made the moves they did.
2. Try to play stronger players. Ask them for advice after the game on what mistakes you made and how to improve. However, remember that their advice may not be correct!
3. Learn a specific opening such as the Ruy Lopez. Play through master games that use the same opening at Chessgames.com. Study books that are at your level on it. Keep using the opening for months or years until you are comfortable with it. Most players only need to know 1 or 2 openings for white and black each. Use opening databases at Shredder and ChessOK to find out popular opening moves.
   
   
   
   
   
   
4. Watch help videos such as these that teach different chess openings at Youtube. Watch other help videos on other parts of the game such as endings, traps, tactics and positions such as the ones by Jrobichess.
5. Learn from your mistakes. Save your games so you can analyze them later. Learn how to record them on paper here and here. It would be best if you review these games with a better player, or chess software that can analyze and explain where you went wrong. You can download a printable scoresheet here and here.  
6. Before you make each move, stop and  look for captures, checks, and threats by each player. This will greatly improve the quality of your game and reduce mistakes.
7. Read articles by chess coach Dan Heisman on how to improve your chess.
8. Play fast games (with a clock) as well as slow games. The fast ones will let you play more games in the same amount of time and practice tactics. The slow ones will let you think more deeply about your moves.