KISS
Keep
It
Simple,
Stupid
Ok, you are not stupid, its just a phrase. I went to 4 proms (2 jr, 2sr). Be polite, be clean, get a haircut (not short or long or whatever, just taken care of).
Then go talk to the girl you want to take. Talk. If you have never talked to her before, then she has no reason to go with you. And, after 5 min of talking to her, you find out she is an idiot or her laugh drives you up a wall or whatever, then you won't have wasted the prom on her.
Next, if you can ask her to the prom, ask her to the movies. Take a night to get to meet her before you go to the prom.
BUT, ask for both, movie and prom in one shot, like "Hi, [girl name here], I want to take you to the prom, but since we have not been out much before, lets go to a movie first." Either she says no, and you go ask someone else, or she says ok, and you charm her at the movies so the prom date is less awkward.
As the Butthole Surfers say, "It's a funny thing about regret. It's better to regret something you have done, than something you haven't done."
It will always be better to think, "at least I asked [girl name here] to the prom" rather than torture yourself with " [girl name here] might have gone with me if ONLY I had asked her!!!!".
Well, you get the point. Worse thing that happens is she says no and in 6 months you are in college with much bigger worries.
But at least you asked!
