Every girl is different, but the best that works with the most of them is to offer flowers and/or write a poem, be confident (you have to understand that even if she refuses it's not a big deal), dress decently, be clean ...

Tell her that you'd be honored to go to the prom with her, and if she'd like to go with you, Keep it simple and direct.

Try to findout if she already have a date, dont ask her in front of a groupe and if you dont get an immediate answer, then the answer is probably No.

if the answer is no, take it gracefully and tell her "Thanks anyway."