ableton is truly a blank canvas in terms of what you can do with it, and using it for DJing will take a little bit of experience and forethought about what you're trying to accomplish with your DJ-set. as a result, there are many different types of ways ableton is set up to accomplish what ever the dj wants to do.

if you're set on using ableton, you should start here with this dubspot tutorial. this is pretty much the bare bones of an A->B dj set up.
however, if youre trying to accomplish an A->B dj set there are other softwares, such as traktor, are set up better for this.