How to be comfortable enough to have sex?

I want to have sex with one of my guy friends. He's a senior and I'm a sophomore. He's really cool and he changed my life. Anyways, I know how to prevent unintended pregnancies and STIs and such; those aren't the problem. The problem is that I don't know how to feel comfortable in my body long enough to have sex with him. In the words of a person from school, I'm "fat, flat, and have no *ss." I also have stretch marks and other skin problems all over my torso and thighs. I know you're thinking "Turn off the lights" but I'm not allowed out at night and my parents actually make sure I'm in my room every night. So we'd have to do it during the day when you can see everything. I really do want to do it. I just don't know how. Also, the other girls he's slept with before are really pretty and sexy so I feel like he's only sexually attracted to those types of girls but I'm not sure.