I agree with Adriana. Also, ask her what she hopes to achieve from her sexual experience. If her main goal is a mind-blowing orgasm you could try using a small vibrator on her clitoris while penetrating her.
If, as Adriana said, you have a big penis, maybe prolonging her foreplay or giving her oral sex would help relax her and her vaginal muscles.
In summary, ask her what she would like.