Approach it with a sense of humor.
Not to laugh at your partner, of course. It's more to recognize the absurdity inherent in such situations. Embrace the fact that it will be imperfect. In a way, it's a skill like any other. You haven't really practiced it yet!
So if anything goes a little cockeyed (pun intended), it's totally okay.
Of course make sure you have protection and you have backup in case a condom breaks or you want to do the deed more than once. Be kind and gentle with your partner; it's her first time, too.
Also, recognize that for a lot of women, just vaginal sex isn't enough to get them to orgasm. There is nothing wrong with this. It's exceptionally normal. Concentrate on foreplay as it might bring her to orgasm (I have no idea what she likes, so you're a little on your own there).
Don't be afraid to talk. "It feels good when you ___" "Can we try ___?" "Do you like to ___?" "Yep, that's the spot!"
And when you're feeling anxious, between now and then, think of all the good things about her. Why you really like her. Why you trust her. Why you think she's special. How good a person she is.
Anyone who is liked and trusted, and special and good as that is also going to be understanding.
You'll be fine.