not used garageband a great deal but i would also say use it as its free with your mac 🙂
just drap n drop the mp3 (or whatever it is - as long as it's not got drm on it) into garageband..
it will put it on a "timeline"..
then just drag another mp3 and put it on another timeline below it and drag it to the desired position you want to start the "mix" at..
afaik you won't be able to do any beat matching, although i think you can change the tempo ..