Is your essay to focus only on the prom night, or can you include background events? If it's alright to include the lead-up to the prom in your paper, start by describing how you were invited to prom, what you felt when you were asked, how you feel about your date for the night, what the reactions were from your friends and family as you told them of your plans.
Continue by describing the preparations. What was it like to look for your prom dress - exciting? confusing? overwhelming? This will be a great opportunity to use lots of adjectives to describe the variety of looks you considered and rejected as well as the final purchase!
If you can't cover any of the background in your paper, you might start by setting the scene for the night itself. What did the room look like? What music was playing when you arrived? How did it feel to walk in, wearing your special dress and shoes, with your fancy hairdo and flowers? How did your date look? Include descriptions of your friends and how they looked or acted.