definetly relax. you'll learn each other over time.
i definetly vote for missionary position to get used to things. jumping from position to position the first time isn't going to teach either of you too much about the other.
if she wants to be on top, let her, but if she's moving in a way you don't like, grab her ass tightly and slow her down or stop her, guide her how you like it.
this is probably the best advice i've ever heard when it comes to sex - granted, you have a penis, she has a vagina, 2 drastically different body parts. however, they're meant to interact with each other. if you're thrusting into her and it feels good to you, chances are it feels just as good for her, and vice versa if it doesn't feel good. i've used this way of thinking many times when with a girl i wasn't "in tune" with. get in, do what feels good for me, and watch her signals. there's been times that i've been plugging away thinking "yeah, this feels good" while in the meantime, she's wriggling around, eyes rolled back in her head, and fingernails dug into my back cause she's going wild.
patience and take it one step at a time. i doubt you were master pussy licker on day 1, and even if it was good for her, i'm sure you've gotten better on her and know what really gets her going. same with sex.