A lady introduced me to this joy, and at first, she just teased with a finger, around there. It's all about relaxing, and wanting to enjoy her penetration play!
It took a few times, and there is always the fear of, what if you shit!
She made that part of the foreplay, enemas, cleaning out, later.
Having a woman loving you, penetrating, anally, is so AMAZING!!

Both of these guys have great suggestions, but Ill throw my little bit of anal wisdom in to: The number 1 most important thing is to relax. If you're tense or apprehensive, its going to be very difficult to have anal much less enjoyable anal. Heres my advice, first make sure you are relaxed, in a good mood, and with someone you trust. Second, one you have the details planned, take a ice relaxing shower and take your time douching until you feel you're clean. When your wife or partner starts playing with your ass, communicate. Let her know when she is doing something that feels good and also when she doing something thats not. When she starts inserting something in your ass, you need to communicate what you are feeling. Here's a very common issue, your asshole is protected by two ring muscles. The one you can immediately feel when you run your fingers over your hole is the outer ring which you can control. So you can make it open or close if you want. The second ring is about 2 inches behind that one and you cannot control this one as easy. Its an involuntary muscle. ALot of problems happen here because you're horny and want something in your ass, so you relax the outer ring muscle around your asshole. All good until almost immediately after that, you encounter the second ring and its closed tight and really doesn't care if you get fucked or not. a lot of potentially hot ass sex stops dead here. But if you relax, take long easy deep breaths and try to visualize that ring muscle, you can make it relax and let your partner insert what they want in your ass. Last but funny thing is once you break through the second ring, your ass will be wide open, like its done this a thousand times before.

Start small with fingers. Consistency, relaxation, and small incremental steps. Let your wife have a finger party after you cum. You'd be more relaxed. Once you get gains don't get greedy and skip the gradual process because it will hurt. After a while you will learn what muscles you can manipulate a the one's you shouldn't. Patience is a virtue...and a good safety tip is always wear eye protection while receiving oral and prostate stimulation at the same time because you will blow the back of their head off.

First of all, you are not alone. There are many people whose ass is so tight that they cannot take a dick or dick size object. This is well within what is normal, so don't fret about it.

Having said that, it's clear from your remarks that this is something that you really want to do. So let me give you some tips and words of encouragement.

The most important thing is learn to relax. This is not especially difficult to do but it does take some practice. The normal condition of the anal sphincter is to be somewhat tensed to keep the opening closed. But with thought and concentration one can learn to relax these muscles to allow entry. And take the entry slowly. It's an unwritten rule of anal sex that the person being penetrated gets to control the entry. At the first sign of pain, stop, relax, wait awhile, and start over. Or even postpone it for another time. Otherwise the pain will cause you to tense up which will cause more pain and lead to a never ending cycle of pain and tension.

Another important thing is to stretch your asshole. Looking around the internet it's not difficult to find photos of (men mostly) who can take enormous things up the ass. If you wish me to send you some, just ask. Popular items are a two liter beverage bottle and of course a fist. Full disclosure here, I once fisted a guy almost to my elbow so I know it can be done. I did it not because I wanted to but because he wanted me to. Do you think that these men were born with a huge asshole? Of course they weren't. They got it by stretching it out over a period of weeks or months. If they can stretch out their assholes to accommodate a two liter bottle or a fist, you can stretch yours to take a dick.

Good luck and let us know how things go.