So I'm gonna assume you've read some guides on How To Get Fucked Up The Ass; use lube, go slow, condoms are non-negotiable. And if you haven't read any Bottoming 101 guides, now would be a great time.
What none of those guides spend much time talking about is the importance of comfort and feeling safe. Taking a dick up your ass isn't easy! And the thing that makes it the hardest is if you're with someone you haven't developed trust with. A lot of the difficulty, especially with virgins, is anticipating pain and being anxious about it. If you're with someone you can trust, you'll be way more comfortable knowing that if you communicate any discomfort to him, he'll slow down and/or stop as needed.
For example:
My ex was a virgin when we met. He'd tried getting fucked in a hookup shortly before we met--dude couldn't get it in, basically. And believe you me, he wannnnnnnnnnnnnnted to get fucked. But it just wasn't working, because despite being told he was a virgin, the top in question didn't eat his ass, didn't finger him, and barely used lube. In essence, the top saw a smoking hot twink boy and didn't give a shit, just wanted to fuck him.
Cue me. We met, I knew he was a virgin. We didn't even start playing with his ass until we'd seen each other a few times. Then we'd add rimming and fingering to our activities. After about, hmmmm, three weeks or so? He decided he was ready. And he had a great time! Took us a few sessions before he could take me all the way (I'm not hugely long, but I'm definitely girthy), but it worked every time, without pain.
Moral of the story: find you a man who gives a damn about your well-being and comfort, develop trust and intimacy, and go to town when you're ready--not when he wants it.