Well, you've got a date. You lucky thing, you! You're through stressing out about being asked or asking someone. In short, you've got it made.
Is your date your boyfriend? Have you actually brought up going, or is it just assumed? Before you plan too much, be sure to verify that you all really are going. Guys can be weird when it comes to dances, and you need to know if you're going to have to drag him to the prom or not.
Best wishes and have fun!