Anything goes in a spin class! I have seen people in full kits to sweat pants to even jean shorts. Wear what you are comfortable wearing. I think most gyms have spd pedals on their bikes, so if you have spd cleats, you're golden!

You won't have your own "regular" saddle, the bikes usually have those big, padded fluffy seats (yuck!), but you're only riding for an hour, so it isn't the end of the world.

You could always go observe before you take a class. Or talk to other people in the gym or your instructor.