I want to "do it", but im ashamed to tell.

So, I have been dating my bf for around 8 months. I really like him, but i have never liked physical contact with people. Anyway, we have been together for a while and i have started to want to do things further than holding hands, kissing and hugging. We are both 18, and i think it's the right time to errrm... have sex. The problem is that i am really ashamed of it, and i don't know how to tell him, it just makes me REALLY embarrased. Also, he might not take me seriously, since i have always been a really, pure, i guess, one. So maybe he thinks I'm joking, dunno. What should I do?