If its for a high school group like glee or a ACapella or so on, sing "God Bless America" or a familiar folk song; they are likely looking for the blending quality of your voice. If its for a solo, pick a song in the genre of performance. And if for a school musical, a song from Oklahoma or any other Rogers/Hammerstein musical will demonstrate your voice and ability. If its for Opera, ask your voice coach (almost all opera students have one) for an appropriate aria. If its for a rock band, I suppose any song they cover would be fine; sing something you know and can find the karaoke for on YouTube.