You are experiencing "performance anxiety" - fear of failure.
Being physically and emotionally exposed to someone you don't no that well can be scary especially when you are not used to it.
Relax, take your time, get to know her, get comfortable, have fun!!
Resist the pressure to perform (as in get an erection and come) and just enjoy yourself and her. Extend the foreplay, this can go on for hours
Oh, and get lots of practice!