The both of you need to talk about it - what sounds fun, what sounds exciting, what is going to far (for either of you). Come up with a safe-word (a word that she can use the moment things have gone too far - and follow that word like it's the Law. The moment you hear it - Stop. No matter what. Oh, and make it something a little unlikely, but easy to remember. "No" and "Stop" won't work. She may want to use those words during sex. Make it something like "television", or "chair" - also, in my book "no, really" works pretty well.")<br />
Other than that - hold her down, grab her hair, **** the hell out of her. Find out if she wants you to talk dirty while you do it. Remember, sex is play. Keep the communication open and fun, and find what gets you both off.
Consider just ripping off your clothes and giving her a safe word to use for you to stop if she fills things are becoming to intense.