1. Learn to smile and be happy with what's going on in your life. Nothing is sexier on a guy or a girl than someone who looks genuinely happy to be meeting someone, doing something, etc. It's not easy to do, and being happy with your situation is, for some people, a lifelong struggle. Looking like you don't want to jump off the Empire State Building is a pretty good way to exude confidence about yourself, though.
2. As others have mentioned, take up a hobby. Perhaps in grade school you had an activity you really enjoyed? Take the time to master it. TV is not a hobby; intellectually stimulate yourself and you'll naturally develop confidence.
3. Learn to not be shy! You may be the most attractive, accomplished person in the room. If you don't know how to talk about yourself and engage in conversation with someone else, though, who's going to know?