Hello, sweety. Just say to her "are you going to prom? I know we don't know each other all that well, but you seem nice. maybe we can go together as friends if you don't have a date already." Clarify immediately how the money would be handled (are you planning to buy her ticket or go dutch?) I also suggest you try going with a group of kids. It's more fun, and will take the pressure off both of you to be romantic if you don't want to be.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?