I agree with those who say to take at least one cleansing enema. You should also wait between 30 and 60 minutes to make sure the water and dirt are completely out of your system before attempting anal play or you could end up with unwanted surprises. I recommend using a two quart bag and taking of the water, thought not necessarily at once. You can take a few ounces and expel when you have the urge to do so. The enema stretches you internally somewhat so that will help later. But if you'd rather not trouble with that, you can take a Fleet enema or two. I also suggest that you not start with anal sex but that he should stimulate you all over and give you several orgasms before you even begin to think of anal.
When you are, I repeat, YOU, are ready for it have him massage your anus gently with a lubricated finger. It often helps if he perform oral sex on you or fingers your pussy while doing this to get you relaxed and aroused. When you are relaxed and want him to penetrate you with his finger, let him enter slowly and gently. Take deep, relaxing breaths as your body gets used to the new sensations. If he goes too fast or too far for you, tell him. Remember, it's your body and experience that it paramount here, not his enthusiasm and excitement. Give him directions of where and how much. Let your body be your guide. When you feel ready to take another finger, tell him to add another and let him massage your rectum and gently stretch your anus by rubbing it with two fingers inside and his thumb on the outside of your anus and perineum. Have him add a pool of lube to his fingers and feed it into your anus to lubricate your rectum. (You can also buy a lube shooter to get to those "hard to reach" places.) To open you up a little more, have him make a V (victory sign) with his fingers internally then removing his fingers somewhat as he stretches out his fingers. If you want and think you can handle a third finger, let him know and have him partially remove his two fingers and add a third one. Relax and breathe out as he starts to penetrate you, guiding him all the way to stop, slow down, go deeper or not so deep. Don't feel like you're being a pain in the ass by telling him what to do. Most men love hearing a woman say what she wants in bed and it will only excite him more.
Tell him to be patient and to let you guide his movements. When you feel you're ready for his penis, get into whatever position is most comfortable for you. I prefer to have a man kneel or get on all fours behind me then I slowly sit down on or move onto his well-lubricated cock. Tell him not to push. Think of your anus like a hungry mouth beckoning him to enter you. Slowly. Slowly. Slowly, push your anus against the head of his lubricated cock. There is no such thing as too much lube when it comes to anal. Move against him, again, don't let him thrust, just relax and open up to him to take more inside when you're ready to do so. Keep taking deep breaths, easing his cock into you, millimeter by millimeter, if necessary. This is all about pleasures to be attained, not a goal to be reached.
If you need a few minutes between pushing to get accustomed to his length or girth, take your time. Hold still and take deep breaths. You will probably feel his cock contracting while you do this. It will puff up and stretch even as you are holding still. Just work through it and breathe. Slowly and gently back onto his cock whenever you're ready to take him in and stop as often as you need to. The first half is the hardest to take as the head is typically larger than the shaft. Once it's in, you'll feel and sometimes a popping sensation. When you feel that, you know that the rest of it is just length, which is usually much easier to accommodate. Just like a baby being born, once the head and shoulders are out (well, actually IN you) the rest slides quickly and your anus clamps shut tight against him. He might feel the need to thrust but he must wait until you are ready. When that happens, start sliding up and down his shaft at whatever pace is comfortable for you. After a few minutes, your body will realize the "worst" is over and will relax, allowing you to move quicker on his cock to take in his depth. By this time, I'm usually so turned on I'm ready for him to take over and continue the pace and sometimes even thrust against him myself while he's thrusting into me. It's very intense but time for that later. Have him move slowly at first and when you're ready for him to go harder, let him know.
It's best for him not to completely remove his cock from your bottom. When the head pops in and out, it can be painful, sometimes even for the most experienced ladies. Just the shaft alone feels amazing and your tight bottom is all the stimulation he needs so there's no need to exit you completely. Either you or he can also stimulate your pussy and clit while you're having anal sex, which makes it all the more intense. From then on out, it's just relax, relax, relax and enjoy.
Afterwards, you can use some Preparation-H suppositories, as Petal suggested. If you are prone to hemorrhoids or rectal pain, you can use Preparation-H cream or ointment as all or part of the lube, as well.
Above all else, make sure that anything that touched the anus does not get in the vagina until it's been washed completely. Make sure you wipe front to back when cleaning up and that the lubrication does not go near the urethra. You can transfer bacteria and end up with a ferocious urinary tract infection if you're not careful.
So use plenty of lube, relax, take deep breaths, move against him, not the other way around, play safe and have fun.
Dianne