Relax, have fun, take things slowly, have fun, never say anything remotely negative (unless something hurts, but just yelp "ouch!" and she'll get the idea, I hope), have fun... you get the idea. Constant communication is absolutely necessary for having fun... that way you always both know where you stand, and what works or doesn't work. If she's uncomfortable with something, don't do it. Let her decide the pace, for the most part. If she feels completely at ease with you, the adventurousness will come on its own... so don't rush it.
And don't be afraid to laugh (not at her, though)!!
People take sex too seriously sometimes.