As a general rule, you will not have any problems. Yes I know everyone says there are bacteria in the rectum, but this is usually a cop out plea. There are more, (and more dangerous) bacteria in the human mouth than in the anus, yet nobody knocks oral sex. (figure that one out) Take lots of time preparing her, and getting her gently and very gradually stretched open to accomodate you. After entry, you should let her set the pace. You be very slow and gentle. This can be the best or worst thing she or you have ever experienced, and it all depends on YOU and your technique. I have had anal with 8 different women in my life. 7 of them had what they described as "The most powerful orgasm they ever had in their life" (their words not mine) one just didn't stick with it long enough.
Several of these had never tried it before, and became ADDICTED afterwards, requesting it everytime we had sex. So do both of you a favor, and do your best work here so you will BOTH enjoy it for the rest of your lives.