Relax, we've all been there.
I, too, think it's important that you're upfront with her and tell her that it's your first time. If it's not her first time too (I'm assuming it's not), she'll be able to take a lot of the awkwardness away from the situation. Plus, she'll be able to help you in the right direction, so to speak.
After all, she wants it to be good for herself as well as you.