what software

[kindelsesh]

Virtual Dj or Traktor.
Both are simple to understand and easy enough to use people also suggest using Serato is another good software but i've never used it so wouldnt know much about.

Good thing about Traktor is you can get a few different versions of it, theres LE which you can get free with a number of different audio interfaces, Traktor Pro, and then Traktor Scratch Pro.
I'd check this link NATIVE INSTRUMENTS : HOME | HARDWARE AND SOFTWARE FOR MUSICIANS, PRODUCERS, DJS, GUITAR AND BASS PLAYERS

[teknorich]

Get yourself the Traktor demo and have a play with it:

TRAKTOR PRO Demo Download Mac and PC

Virtual DJ is a simpler interface (and bizarrely seems to synch tracks better, despite being cheaper software) but overall is a bit more "kiddie", tho might be a good starting point.

[Barely Human]

Traktor all the way