First you must be checked out by a doctor including rectal exam and colonoscopy. If you have a fissure or tear in your rectum, it can lead to infection or even cancer if untreated.
Once you are medically cleared, here is a reasonable way to have pain free anal sex.
1) Give yourself an enema. Being clean is essential Your partner can help you with this and is a good introduction to intimacy.
2) A patient ,willing partner with a small penis to start is best. Dont start with a big penised partner, please ! Explain that you need to proceed very slowly. There is nothing wrong in telling her you are new and a bit afraid, If she won't acomodate you, just take her to lunch, but be nice !
3) After obtaining xylocaine topical anaesthetic liquid from the pharmacy, apply a small amount to the external and internal tissues
of the anus. to slightly numb the tissues to allow insertions.
4) Have small , medium and large butt plugs available. Have your partner insert the small butt plug with plenty of anal lube slowly, with a twisting, gentle in and out motion
(anal lube is available in most adult book stores as well as butt plugs and even benzocaine numbing gel )
5) Have your partner experiment with a heavily lubed finger or two. Start with the smallest pinky.
6) Now its finally time for your first penile rectal insertion. It is best to start with the doggy position with your head down on a pillow and your ass in the air with your knees tucked close to your body
7) With your partner behind you with sporting a heavily lubed condom, take slow deep breaths as the penis is inserted slowly into your rectum. If there is discomfort, have her withdraw a little , and try again. Slowly pumping deeper and deeper is the best approach to get used to the fullness of a penis in your rectum
Once the penis is buried to the balls, have her hold there for a short time. Then slow deep pumping will open your anus even
9) The rectum can not easily handle hard fast pumping, so try slow deep pumping.
10) Next, with her sitting up against the bed post, sit down on her penis facing away from her, again with slow deep pumping.
11) Finally, lie on your back with your partner facing you, and insert her penis facing you. This allows for kissing and seeing
12) If she can, have her cum deep inside you of course with the condom on. Many can't come in the condom as they
require hand stimulation.
Once you have learned the niceteze of being topped, you can experiment with larger, more agressive shemales. Another advantage to learning how to take a penis deep and hard, it to have a session with two or three shemales.
Dressing up in cross dresser clothes will also get you in the right frame of mind with your prospective partners.
Hope this has helped.