Everyone has a first time and even after you get it in you will spend the rest of your life finding ways to make it better.
If you are on top you might try it with him lying and you sitting on top of him. Then you can position his penis at the entrance to your vagina while you lower yourself down on it. This way you have control over how much pressure and how far. If he has a foreskin it might work better if you/he retracts it a bit so the tip of his bare head is going in.
If you want to allow him control it might be better with you lying and him lying on top of you. You can also try this position with your legs over his shoulders making your vagina more accessible.
Relax and enjoy.