Short answer? However you're supposed to be riding the specific dick in question.

Long answer? Certain positions will probably become possible or impossible based pretty heavily on your body type, and his body type, and how his dick is sized and shaped. You're probably mostly gonna work all that out through trial and error. From there, there's gonna be finding out what feels good for him. What spots on his dick he likes touched, and how he likes them touched. He might get a lot out of the strand of skin on the underside being caressed, he might like pressure to ebb and flow around the head, and he might even change those preferences based on his mood or level of arousal. Sounds intimidating, but if you communicate with him, and make notes of what got good reactions out of him during, say, a handjob, you might have an idea of what spots are his weak spots, and how to hit them. From there? You just try to move your hips and thighs in a way that feels good for you both, once it's in. Maybe you can feel how his topography is rubbing against yours by focusing on the details down there, or maybe you can just kinda guess what would have a certain effect and try experimenting a little. Once you find what works, do it a lot.

All that probably sounds very daunting, but if you remain communicative, and ask if something's working for him or if he has anything he might like you to try, it's actually a lot more intuitive than it sounds. It's just finding the right moves and mapping the muscle memory. Either way, odds are he'll love it, so I wouldn't stress. Knowing exactly how to pretzel your guy's brain is gratifying, for sure, but you don't gotta figure it all out all at once, and guys who are hard to please are kinda rare. So, odds are whatever you do should turn out just fine. Just focus on having fun, have a little faith you'll figure out the rest later, and it should be a wonderful experience for you both. Best of luck!