If I was in London I'd show you... maybe give me a ring some time?
It's easier than it looks! I was confused for years and just did it all in the arrange view
Basically what you do is:
1. Go to session view
2. you've got vertical "lines" in front of you - each of these is a "deck"
3. Make sure you've got 2 or 3 of the vertical lines / decks
4. load the tunes you want to mix into the "slots" in the "lines" - just click in the slot and find the files you want to play. Helpfully there's a little "play" icon next to each one - that's what you click on to get them going
5. BUT before you get started, you have to set warp markers for each track. This is the secret of good djing in ableton. Instead of putting time into knowing your vinyl records so you can mix them, you need to put time into making sure the warp markers are right - so you can mix them.
6. To do this in a quick and dirty way pull the "tabs" that enclose the file so the intro is hidden and the main bit of music starts with a downbeat / first bar of a 16 bar section. Make sure there's a yellow warp marker there. You want a few yellow warp markers throughout the track so that ableton can pick up the beatsof the track. With luck you should be able to set the first one, then have a look near the end of the file and put in a warp marker if there's not one there already. drag that so that it lines up with the right beat. (This can be a bit tricky - especially for reggae

)
7. DJing then consists of clicking on the play icon next to the track and fading in the next track - in the vertical lines there are icons marked "1" and "2" which when selected put them under the control of the cross fader - wiggle the cross fader on the screen and you can fade between the tunes.
I've left out a lot here (like monitoring!) but it's not that hard to figure out.