Don't worry, everyone feels just like that. Soon after, you'll be laughing at how much you worried . Just listen, watch, and do what is asked of you...also, ASK. If you don't know, ask. You may think it's a stupid question, but I'm sure they've heard worse, and there's no such thing as a stupid question. You'll feel a lot stupider if you DON'T ask, and then find out you did something stupd .
It is better to ask, then to make a mistake.