A few suggestions:
If your thinking of a suit:
1) Buy a suit that you can use more than just for your prom, so think of something like a charcoal of navy. If you want to get wild or match your date, do it with your tie because that is cheaper to change (or get rid of) than your entire suit. I don't think you will regret purchasing a garment that you can wear again in the future.
2) Seeing as how you are still fairly young, entering college, and most likely won't require...