Your favorite song would work! Or maybe look up the top songs at the moment! If you have a really good voice, maybe an Adele song? I also like the song Jar of Hearts. Any song is good for a talent show!
It all depends on what kind of voice you have. If you have a really strong voice, then you should sing an Adele song. If you have a lower voice, then you might want to go with Hey Soul Sister by Train.