Start by reading the "Uplifting Material" section in the DJ bible. There are some great posts from Pook and others that really get you motivated. It makes you determined to fix up your life. Good stuff.
After that? Get more friends. Talk to more people. Be active in class. Get into some kind of sport or club. Do stuff on the weekends. Learn how to drive. Do something you're good at.
And above all else: Don't let people get to you. There are a lot of stupid people in the world, but don't let them bother you. Getting all agrivated only stresses you out. Stress brings you down in all aspects of life. Its just not worth it. The bullies don't matter, they're just scared little boys on the inside. The problems all have solutions, and all problems are temporary if you can fix them. This applies to girls as well. So what if you get turned down, or even straight-up denied? Don't let it get under your skin. You are the master of yourself, don't let anyone else take over.
.....I'm not sure that last part relates to building confidence, now that i read it again.... but hey, its good advice anyway, so it stays.