If your rating is below 1200...
International Master Jeremy Silman is a regular columnist on chess.com and the author of many superb chess books, including The Amateur's Mind and Silman's Complete Endgame Course. In The Amateur's Mind, he writes:
Don't play with fear in your heart. If you play with courage, the worst thing that can happen to you is a loss. Since we will all lose many games in our lifetime, we might as well go down with honor and make every game as instructive as possible. Playing passively and getting routed is no fun at all and teaches you nothing.
Below is a link to an article (from his own website) that he wrote about improving beyond beginner level. If your rating is below 1200, give it a read and put the ideas into practice with every game you play.
[Note: USCF tournament rules have changed since Silman wrote the article. Now players must record each move after it is made (see the revised Rule 15A).]