After reading everything there is a lot of good and bad info in here.
Just bring it up and talk about it like adults. Say its something you want to try and will take it slow and listen to her. She HAS to trust you. Now, if she agrees do the following.
1st: Go buy some Anal-eze. You only need to use it the first few times until she gets used to it.
2nd: Buy a good quality lube.
Now, when it comes time to "do the deed" take it easy. This needs to be very relaxing for her, I suggest candles and light music. If she smokes, great, toke up. If she doesn't then have a couple drinks. ONLY A COUPLE! Don't want to numb her up too much or some girls can't orgasm. Foreplay is going to be big here, take a lot of time kissing a caressing before you even get undressed. Seriously guys, spend like AT LEAST 10 minutes on this. An alternative to this would be a nice shower together, just give her a couple extra minutes alone to scrub her ass.
Now that you're naked and in bed keep up with the foreplay, don't rush it. You want to build her anticipation. If she likes receiving oral then make it the best she's ever had. Even from time to time running your tongue over her hole will help get her used to it. Now, before you stick ANYTHING in her ass get her off first, at least once. It'll help her relax and she won't feel you're being selfish.
Now that you've gotten her off grab the anal-eze and while eating her out or something that's giving her pleasure slowly push your finger in and spread the anal-eze. Once again, DON'T rush it she isn't numb yet, the stuff takes a little time to work. In the mean time grab the lube, lube up your finger and get her used to you slowly using your finger. You can, but you can't really use too much lube.
Now that she's properly lubed and relaxed lube up your dick, once again use a good amount. Now especially if this is her first time you need her somewhat distracted with something like a clit on her vibrator, this is her job. You're job is to take it slow and easy. Depending on how tall each of you are, body mechanics etc... there are a couple positions that work best.
First one that seems (in my experience) to be the best when its a girls first time is for her to be on her back, using a vibrator on her clit. Now you can help the angle if needed by placing a pillow under her lower back. Take it slow and easy, let her tell you when to speed up, slow down, go deeper, go shallower, etc... Communication is HUGE at this point!
Second one that is always linked to anal of course is doggy style. Same principal just a different angle and can be more painful for a first timer.
Once she's used to it the sky is the limit though.
Every girl I've ever done anal with has always liked it and after a few times REALLY liked it. I've had girls scream so loud I seriously thought I had hurt them from being too rough or something but it was because they just had the most intense orgasm of their life.
You can even take it further once she's more used to it and use a dildo/vibrator in her pussy while you fuck her in the ass. Once again its one of those "build up to" kind of things but holy shit she will cum like you can't possibly imagine. Plus, if you do happen to have a vibrator in her pussy and you're in her ass those vibrations transfer through that thin little wall and rock your world too.
Now to really fuck with y'all. If you aren't willing to have her stick a dildo in your ass, don't expect her to let you stick your dick in hers. Just be prepared that if you fuck enough girls in the ass for long enough this topic WILL come up and if you aren't secure enough in your own sexuality to let her do it then she'll cut you off.