Definitely read the manual, particularly sections on Concepts, Session View, Arrangement View, Clip View, and Tempo Control and Warping. And here are some tutorial’s I found with a quick look around:

• Video: DJ’ing in Live Part 1
• How to DJ with Ableton Live

They’re both pretty basic, so have a look, try some stuff out, and then dive into Ableton’s own tutorials. The things you’ll need to get a handle on are:

• warping
• crossfade
• zooming / moving around in the clips and the arrangement.
• adding effects
• moving between clips in session and arrangement views
• recording

(this is part of a my upcoming tutorial on KJ’ing in Ableton)