Its actually a 737-300, not that it makes a lot of difference! And yes it is fixed base, although mine (as I believe others have been) was in the full motion sim, just without the 'motion' unfortunately
In terms of prep there's not an awful lot you can do. The instructor does a very good job of explaining everything you have to do and everything that is expected of you, as well as giving you a full break down of the instruments and controls you'll be using.