Wow - trust me, we're not all having great sex, in fact a lot of us are working it out as we go along!!
Can I just say, it's worth talking to your boyfriend, because once you open up to him about this, not only will he understand what you are feeling, and he can help you with it, but then you don't need to worry what he might be thinking. Talking about something like this will create intimacy and allow you to feel more comfortable about it.
I also suggest that you learn to accept your nakedness. If it's body hangups that bother you, then start doing something active - I'm not talking about trying to become a gym-honed Gisele - the reason I say it, is that once you start using your body, you feel better about it, and you feel sexier and more confident.
If it's nothing to do with your body per se, and you are just uncomfortable about the sexual side to it, then try to relax about it slowly. Sex is natural, and it can be so much fun, and you are in charge of your body, you set the limits, and whatever you choose and like is not wrong in any way. People like different things, and you are allowed to let go, and enjoy sex. I am sure the idea might be off-putting, but have you considered using a vibrator? Most women I know have one, so don't think it's a "dirty" idea - it helped me to gain sexual confidence, once I learned how to please myself and have control over my body sexually. My boyfriend now likes to use it, and it's not a threatening thing to either of us, it's just a fun toy.
But if you would rather keep things simple, then start by talking to your partner. A good game to play is to have him get naked, close his eyes (or use a blindfold) and then you touch his body wherever you want, and tell him what you like about it. Then he can do the same to you. Or vice versa. The idea of this is that you can get to know each other's bodies without feeling like they are looking at you looking at them.
Ignore the media, and ignore porn, and ignore what you see in movies. We all have sex, and very few of us are movie stars or porn stars. Sex is about the two people doing it - ie you and your boyfriend! - and it is all about what the two of you like. This is where the assertiveness can come in. Start telling your boyfriend what you like - don't be shy - and it won't be long before you start telling him what to do.
Have fun.