Decide what you're going to do. You can sing, dance, play music, do acrobatics or magic. Explore things you've never tried just to see if you've got the talent.
Videotape yourself dancing or doing acrobatics or magic. Record yourself singing or playing your instrument. Be as critical about your performance as the show judges will be.
Choose your clothes carefully. You don't want to look like a floozy, but an evening dress isn't exactly appropriate if you're planning on juggling.
Stage a dress rehearsal in front of family and friends. Perform for needy children, at nursing homes and hospitals and at a friend's party. Watch your audience's faces for their reactions.
Get to the show early so you can get comfortable with your surroundings. Make friends with staff and other performers. Respect those you're competing against and you'll have more fun.
Perform with only one goal in mind, to have a good time. Nervousness will ruin the performance more than anything else. Enjoy yourself and your true talent will shine through.
Celebrate with friends, family and other participants. You've just gone through something few people have the guts to attempt.