Nervous about ....this is unusual please help haha?

Ok, so I've had many times, but I've never been in a situation like this. A girl I know is coming over later and after talking on the phone, it is pretty clear we're going to have . I have never been so nervous in my life. Usually I end up having with a girlfriend, or something totally spontaneous. I cant take this anticipation...what should i do to calm down?