I concur with @gailcalled. Don’t make love until you feel really comfortable with your partner. It’s not something to do just because other people do it. If you feel like you don’t want to, then don’t, and don’t let yourself be muscled into it, either.
If you’re not comfortable, there are several things that might help. First, inform your friend that you are new and you really want to go slow, and if he pressures you, you’ll be right out of there.
Second, read books or research it on the internet. Find out what other people do the first time, and how they felt, and, well, how it is done. Practice on yourself, and then, if you want to, practice with your friend.
If you have low self-esteem, it is understandable that you might be reluctant. Usually people with low self-esteem can’t imagine anyone else liking them or accepting their bodies. I’ve been like that. I couldn’t imagine anyone actually wanting to sleep with me. Even after I’d done it many times, I still felt that way.
What I need is reassurance. I need to know my body is not repulsive and that my sexual apparatus is acceptable. Maybe even beautiful. I’m so certain that everyone is better looking and better endowed than I am. Check out the discussion on penis size if you want to be depressed—women seem to be “size queens,” so to speak. There are plenty of such people in the gay community, too.
I think that if I were young again, I would want to tell my inadequacies to my partners. If they still wanted me, after that, I would feel more reassured. I would also tell them that I need constant reassurance—about my body, and about me. I hope it gets better for you; I’ve been having this problem for nigh onto 35 years.
Good luck!