First: you should practice SLOWLY inserting small diameter round things into your ass-hole using some lube like KY Jelly. Make absolutely sure these objects are CLEAN, and that they can't slip out of your grasp and disappear up into your ass! The Urgent Care folks tell stories about people who show-up with all sorts of things lost up inside their ass-holes.
As you get comfortable with, say 1/2-inch in diameter, get something a little larger. Work up to taking things in larger diameters until you can take at least 1-1/2 to 1-3/4 inches and you should be able to handle most average size cocks. These things only need to be long enough that maximum 6 or 7 inches max go inside you. But start with only a few inches initially.
Usually I have a little bit of pain upon the initial entry of a hard cock into my ass-hole as the head spreads my sphincter open, but with enough lube, and if he stops after the first inch or so, to let my ass get accustomed to the new invader, the pain goes away after a very short time. Once the pain is gone, it feels so erotic, sexy and naughty that my mind just kinda goes: "oh, yeah, fuck me with that cock!"
The whole idea in my head that here I am, letting a man slide his cock into my ass and fuck me as if I am a woman is overwhelmingly sexy. I only get to experience it maybe once every month or 2, which is not nearly enough to satisfy my horny hole!

Receiving anal can be an amazing experience, depending on the size of the cock. I've been fucked by a few small dicks, and it wasn't fun. I might as well have fingered myself. I've had my arse stretched by my fuck buddy Dave's thick cock many times, and although there's a little pain sometimes, the pleasure more than makes up for it. I recommend it.

Its amazing. Have someone just pound into you feels right especially when it hits your prostate

It's definitely one of the best sensations you can feel