For the first time, all cocks are going to seem really big. It's been many years, and many cocks ago that I had my first time with a guy, and I can remember it hurting a lot. He was really thick, so that didn't help matters much. I ended up making him stop because it hurt too much, and he wanted me to suck him off. I wasn't really in the mood to suck a lubed-up dick that was just in my ass, and was really more interested in what a pain in the ass (literally) sex was looking to be. So I stopped sucking his dick, and told him I wasn't really into it anymore. He looks at me and says "You don't want to fuck, don't like sucking dick. Are you even gay?"
This pissed me off, and I felt really disrespected.
I immediately opened his truck door and left the church parking lot.
Hopefully your first bottoming experiences will be much better.
Now when I conquer a huge cock, I like to start by sitting on it. This lets me go at my own pace, and I'm able to relax much more. Use tons of lube. The stuff is cheap, and it helps a ton! Don't use the desensitizing shit, as you want to know if it's hurting back there. If it's hurting a lot, you need to slow it down, relax more, and perhaps try more lube. No need to push out at first. Just start by slowly "bobbing" at the top of it, letting it creep in little by little. Sometimes guys tend to try and start humping from below, and that's the time to tell them "stop that"! Just take it slow, remember to breathe slowly and deeply, and make sure he doesn't go too fast for you.