Akari Lee
I am in the 12th grade... and i am going to do a 12th grade project! it will be a manga.. but how do i start one!!! its due on march!!
First, take a few deep breaths. No use panicking this early on; it'll just make things that much more difficult.
Now. You say you want to do a manga -- I'm going to take a guess that you are at least somewhat familiar with the form, whether it be through Watase, Toriyama, CLAMP, whoever. If you're focusing on manga specifically because of its (often debatable) difference from "American" comics, go back and take notes regarding panel setup, pacing, dialogue, et cetera.
Do you have a page minimum? Consider Tokyopop's Rising Stars of Manga contest -- each time they have a three-month period for applicants to complete a 15-20 page manga. That might not be a bad goal. You'll have to focus on a shorter story in that frame, though. As with the written short story, focus on something simple and straightforward, with only one or two main characters. Don't go for epic UNLESS you can do a simple, straightforward, standalone episode. (If you want an example on the American manga side, take a look at "The Calling Grounds" in Rising Stars of Manga vol. 3. It establishes itself as a standalone story, but you can also tell it's part of a larger mythos.)
Feel free to browse among the threads here -- I especially recommend the Pot of Ideas and the Comic Creators Challenge for practice. Work on developing your story at the moment -- make sure you have a good idea of what you want before you put ink to paper. Furthermore, I recommend the CCC thread for its challenges (well, that's kind of obvious) -- you may get a better feel for the form by working on some of the exercises therein.
Most of all, enjoy what you're doing, and good luck!