Ok first of all, if you've NEVER been in an OR, I doubt, and hope you aren't going to be allowed to scrub. There is just too much going on for someone who has never been there to scrub in.
Mainly don't touch anything, and don't move, pick a spot in the OR and stay there and watch. If you want to ask questions, I'd start off making friends with one of the nurses. They'll probably be really cool about it, and be nice to you and tell you whatever you want to know.
If you are scrubbing in, then all I can say is, do exactly what the nurse or doctor tells you. Do it slowly, and if you didn't hear what they said for sure, ask! Also, as long as you are wearing sterile gloves/gown keep your hands clasped in front of you at chest level. Be aware your back or the back of others in sterile gowns is not considered sterile. So you can not scratch your back lol. If you touch something outside the sterile field and then touch the sterile field, you'll likely never be invited back lol.
Mainly just be cautious and I am sure you'll have a great time. Oh one last thing, it is very common for people to get woozy or even faint the first time they watch surgery. Don't be embarassed if you feel woozy, find a place to sit, or lean against something. Whatever you do, do not fall onto something blue (blue = sterile) .