Okay, first off, don’t let this guy pressure you into doing anything you don’t feel comfortable with or ready to do. If you’re feeling self conscious about being on top, simply tell him that you’re not ready to do that yet. If he has a problem with it then he can go fuck himself.
I know you said you want to do it, but your hesitancy makes me question if it’s what you want for yourself or if it’s what you want to please him… You see?
Do only what YOU feel comfortable and prepared to do. Don’t let him dictate what positions you’re going to be in! You just had sex for the first time! You’re not even dating! Just tell him you’ll get on top when you want to, but not right now. Not until you feel more confident about sex in general.
When you’re totally ready to climb up there (and it has to be coming from YOU and YOUR DESIRE to get into that position) read this post from an anon question I answered a while back.
A girl on top makes it all about the girl. It’s about you and what you like and what makes you feel good. He’s your playtoy and you’re doing whatever the fuck you want to him. This is why I’m advising you to wait until you feel more comfortable with sex because being on top means exploring what feels good for you. You’re setting the rhythm, the pace, gyrating, whatever it is you like. Until you’re ready for that kind of control…take it slower and if this guy isn’t okay with that, then you need to find another guy. End of story.