I'm a slow and deep person - Mmmmm!
You just have to get your body to know what to expect. The first time you try to take something the size of a cock up your ass, the natural instinct is to squeeze down and stop it from entering. Obviously, this is not going to be pleasurable. Like someone said, butt plugs are good to get you used to relaxing those muscles and opening up. And don't get a monster and ram it home. Start small, get comfortable with it, and then go to the next size up.
Once you're comfortable relaxing as you're penetrated, you're good to go! Personally, I love being fucked. I can't describe it, it's just hot as hell.
Needless to say, safe sex only.