Lots of lube is necessary, of course, and it does help to use a dildo on yourself first to stretch yourself out. That makes it a lot easier to take his cock in your ass.
I like to get fucked in my ass in most any position, but one position for the first time might be with you kneeling over him, facing him. Your ass and jis cock all lubed up and you easing down onto his cock. That way you can take it into your ass as fast or slow as you want and pull back up if it starts to hurt.
Up and down movements, gradually easing down over hjis cock until it;s all the in your ass. Then just sit on it like that for a while until you;re used to it and there's no pain, then start sliding up and down for a while, then raising up and letting him start thrusting.
Once you're into it like that, you can change to any posotion you want to try and, using more lube, his cock will slide in easily and he can really fuck your ass the way you want him to.
It works for me!
Good luck and let us know how it goes.
Slipzy