lunashock
Well, the best thing to do is to talk to a professional about it and see if it's clinical depression.
I'm a bit iffy on this. A lot of doctors will just "yup, depression" and load you up with drugs without really paying attention to whether or not it's actually good for you. I remember in my High School, well over 20% of the kids were on Zoloft or something similar. I'm sorry, but depression is NOT that common. These kids just had normal hormonal blues and, because it kinda looks like depression and there's no real way to test for depression, they just get medicated. And being medicated can really hurt you if it's not proscribed properly.
Gokan -- Talk to your parents about it. Ask if there's a history of this sort of thing in your family. If there is, then you know what you are up against. That's when you should seek out a doctor. If not (or just "well second aunt three times removed Sally had a bad hair day once. SHe was pretty depressed about that..."), then you'll just have to wait it out.
I know, it sucks. It sucks HORRIBLY. It's painful and it can sometimes really screw you over (like when you just don't have the energy to do anything so you fail your classes, and then you are screwed over when you get out of it and try to get into a good college).
Some things can help make you feel a bit better. A good diet and excercise (NO potato chips, or junk foods of any kinds) can help, but don't expect a quick fix.
Other than that, nothing but good old will power will help. You have to force yourself to live normally (go to school, do your work, go out with friends, etc....). It can be hard, but you just have to do it. That will stop you from hurting your future, at least. And then just wait it out.
Good luck
smile