I understand your concerns about being a newbie, but you'll find that spinning is pretty straightforward. Just a few suggestions that my instructors try to drill into our head: Keep you shoulders relaxed and "out of your ears." Try to keep you foot flat through the entire pedal stroke. When you come out of your saddle to climb, try to keep you butt over your saddle and bend at the waist (keeping a flat back) so that your nose is almost touching your handlebars. My instructors sometimes specifically tell us to get in that position to isolate our leg muscles, but I've also read that it will better help develop the butt muscles that you use when climbing (both seated and standing) outside. When you're spinning at a high cadence, try to tighten your core.
I typically wear my bike shorts for comfort. I also wear my gloves because the nerves in my hand are easily irritated. That, and my hands get really sweaty. You don't have to use such things, however.
Do you know what kind of pedals the bikes have? If they're like the ones at my gym, they're "two-sided." One side has clipless pedals, the other has toe cages. The clipless pedals at my gym are really hard to get in and out of so half the time I have my cycling shoes on but just use the toe cages. I'd say more people than not just wear tennis shoes.
I, too, would recommend getting there a little early--not just the first time but thereafter as well. It's nice to have enough time to set the bike up correctly and to get warmed up.
Some classes and/or instructors are easier than others. No matter what, just go at your own pace. The instructors are there to guide you, but at the end of the day, it's your workout. If you start to feel overtaxed or something starts to hurt (in a bad way), back of the cadence and/or resistance. Be sure to drink plenty of water. I also like to eat a small snack about an half-hour before class.
It's a great workout. I'm not a "gym" person, but I love the intensity of spinning and it's really helped my road riding. And I don't know why the bikes don't have a cadence monitor on them. I've wondered that same thing. Finally, while you don't have to have an HR monitor, I think its a helpful training tool. I don't wear one but would like to invest in one soon. My instructors swear by them.
Have fun.
Kate
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher