You need a program that will allow you to records and make tracks for remixes.
The easiest and best program I use is Ableton's Live.
It's too easy to mash up songs for remixes as long as you have an accapella and an instrumental. Live does everything for you.
Junk179r is correct, accapella (or vocals) and an instrumental track separated is the key. although depending on what you want to do and what type of music your looking to remix you'd be surprised what you can accomplish by chopping things up and looping parts of it with programs like Reason and Recycle (thats what i used 5 or so years ago, although i'm sure there's better software available now)
in the old days, allot of the 12" vinyl singles would have the instrumentals and vocals separated on the b sides, which made things easier.
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