So as she's riding your cock with her ass, stimulate her clitoris with your fingers. Even if she doesn't come, her overall experience with anal sex will be much better than without clitoral play, and she will be more likely to develop an interest in it for future sessions. Remember the basics of using plenty of lube on your cock. Make sure she's gone to the bathroom before hand as well. The risk women face with regards to anal play is developing a urinary tract infection due to the e-coli bacteria collected in the lubricant that seeps down during sex to her urethral opening.
Young women have lower chances of contracting an infection, but the possibility is always there.
If she's willing, she can do an enema before hand to rinse out her lower bowel, making sure everything's clean. Also, be sure the BOTH of you urinate after sex to flush out any bacteria picked up during anal intercourse. Men can get U.T.I's as well. The key concepts with beginning anal is using lube, letting her set the pace, and keeping one or two of your fingers working her clitoris at all times. If you need more info on how best to tickle her clit, ask her.