How do you get only the vocals from a music track?

I've been DJing for a little while, and am using Virtual DJ DJC version that came with my hercules mp3e2 Turntable controllers and need to figure out a way to get only the vocals from a track so i can put it over another and make actually good remixes, and/or do the reverse so i only get the music part of it. I can't find any free software or figure out how to do it with my software or garageband. please help?