I'm in training to teach spin - and I see you got some very good advice above.
The point about getting there early is key - this time of year there are tons of newbies (10 minutes early is plenty).
No one can tell how much resistance you have on, just go at your own pace. Get a feel for the bike. Hopefully the instructor gives tips on form, try not to lean on the handlebars and feel free to sit down when you are breathing too hard. Everyone was a beginner at one time.
Water and towel are important, I also try and find a bike near a fan.