A talent show is a golden opportunity to show off what you can do, it's also a good way to make friends and impress people that you don't want to be friends with. Even if you don't think you have a natural talent, you can steal the show with a good costume or some creative ideas.
Bad Magic Show
This one is pretty straightforward, put on a magic show but do a terrible job at it. Dress the part, put on a top hat, dress shirt, cape, and draw a fake pencil moustache on your face--bonus points if you make your appearance on stage to "The Final Countdown" by Europe. Then as you begin attempting to do tricks, have them all go wrong; choose the wrong card over and over, reach in to your hat for a bunny and have the bottom of the hat ripped. By the end of the show, pretend to be really irate and storm off stage. If you have an assistant, you can have them reach down and pick up the card you were searching for after you've gone.
Have your whole band on stage pretending to play an Ashley Simpson song, when she starts singing, you don't. Then dance off stage embarrassed that you were caught lip syncing.
Get a few of your friends and put on your own boy band show, and dress as the characters. You can dress as a pretty boy, a bad boy, a jock, or really any caricature you want. You can either lip sync a song by a boy band, or make up your own, making sure to do really poor choreography. This one is funny regardless of whether you have a group of girls or boys pretending to be the boy band, and it pretty much works for any age. Name your group something really cheesy too, like Cool Town Boys.
Doing a comedy act is a great way to be original with your material, and not too many people are willing to attempt it. If you write jokes about your city or state, you'll be sure to captivate your audience as they will be able to relate to the subject matter. If your talent show is at a church or a school, talk about that, or the frustrating drive there, or the music on the radio you're forced to listen to, or really anything you think is funny. You can also replicate jokes from "Saturday Night Live" or some other comedy program. Write your jokes on 3-by-5 cards so you don't forget them.
If you know a lot of people at the talent show, you might try roasting people. Roasting is a simple process, you poke fun at the obvious traits and flaws of well-known people, such as faculty. Everyone will get the jokes and your teachers might find it flattering to be mentioned in your act. You can say stuff like, "How about our English teacher, the way she goes on and on about Hemingway I thought she was going to show up with him today."
Have a fake dance contest with a friend or teacher. Dress ridiculously, dance ridiculously, and have a judge or the audience declare a winner.
Not a lot of people think to show off their eating habits as a talent. Don't do anything special, just introduce yourself and silently eat an ice cream sandwich or something delicious. Take your time. Around the one minute mark, people will begin wondering, whispering, and giggling. When you're finished, simply thank the audience and walk off stage.
Lip sync to the Numa Numa song. The song is actually called "Dragostea Din Tei," by Ozone, but you'll probably remember it from the Internet video sensation Numa Numa, with the Drew Carey-looking fellow dancing to it on webcam. When the song breaks out at about a minute in, have all of your friends run onstage and begin dancing with you. The song pretty much does all the hard work for you.