First of all, you will love those bikes. We have them at the gym I go to, and they are by far the best spin bikes I've ever sat on, and I've been on a lot of them. If you have shoes with SPD cleats, take them. They clip into the pedals on the bike.
As for classes, most of them will be focused on cardio. You won't get a bike-specific workout in them, but you will burn some calories. Occasionally you'll find a class taught by a cyclist (we have a triathlete that teaches classes at our gym, but not very often), and those classes are the most beneficial. But, for me, spin class is about burning calories and working on your cardio, which they are effective at doing.
If you class has fans, position yourself in front of one. Take a towel and a water bottle to class.