Replying to Music Editing Software.

Alright. I'm tying to do some edit's/ remixes/ mashups/ for some music. Right now, I'm using GarageBand with a few jampacks but it's not working too well. I'm not sure if this is because I don't fully know how to use the program, or if the program just doesn't cut it. What I'm mostly looking for is software that will allow me to adjust more on already made songs imported from iTunes. Does any software like this exist, and if so, what is it?