1. There are many ways for your girl to prepare herself for anal sex, as far as cleanieness is concerned. Look up some information online and go with whatever makes her the most comfortable.
2. Next, I don't know if you are of legal age to drink, but a little liquor will help her (and you) relax.
3. Don't PLAN IT. If you do, she wont' be able to relax and it will hurt like hell. It helps if it's spontaneous.
4. The best thing to do is tease that area with the tip of your finger during sex, when she is ready, she will try and take all of it. Stimulating both 'areas' will drive her up the wall and her orgasm will be mind blowing. If the tip of your finger is all she needs to orgasm, great...your headed in the right direction.
The next time you are having sex, tease her again with the tip of your finger, she may want more this time, maybe the whole finger. Next thing you know, you may be using a 2nd finger. Eventually, she won't want your finger anymore, it won't be enough, she will want you to slip it in. The best position is with her on top. Have lube near by, you don't want it to be uncomfortable.
A good alternative to a finger is a small but plug, using the same technique mentioned above. They actually have one for 'beginners'. They are very inexpensive.
5. Once you are done, you should both clean-up, RIGHT AWAY.
Keep people off-balance and in the dark by never revealing the purpose behind your actions. - Robert Greene