Choose your talent. Pick something you do well you're comfortable doing in front of a large number of people. Many a fine singer has been sidelined by stage fright.
Know the audience. Find out who will be there, whether it's parents and friends of the participants, your classmates or co-workers, or a little of both.
Choose a piece that is accessible and familiar. If you're singing, dancing, reading a monologue or doing anything else that requires the performance of a specific artistic work, choose something within your abilities and something that will resonate with the audience.
Select your costume and props. Look sharp but not too flashy; you don't want your clothing to distract from your skill, and you don't want to look like you're attending a costume party.
Practice. No one likes to see anybody up there winging it. Even professional improv comedy troupes do a certain amount of preparation for their performances.
Be professional. Perform gracefully and with confidence. If you make a mistake, work through it; there's a chance the audience didn't even notice it.
Get in and get out. Do not exceed your allotted time, and don't be afraid to fall a bit short of it. Giving an audience not enough of a good thing is better than giving them too much of a great thing. Do your thing, but as they say in show business, leave the people wanting more.