Don't stick it in her ass the first time. Look if you like this girl her first time should be at least a little romantic. Flipping her over a chair and shoving it up her butt doesn't fit that bill.

Do vaginal intercourse first. I have never had sex with a virgin, but I would think it would be best if you played down there for a while first. Just to make sure she is as turned on as possible. I would imagine that would help with the possible pain and discomfort.

Please do not stick it in her ass though. She may say she is willing now, but chances are it won't turn out well.