Questions about Catholicism. Please Answer!?

Okay so I'm 13 and a Catholic. I go to church every week and I don't get this: The Ten Commandments say the rules specifically. There's really no detail to them. Like in you shall not commit adultery. That's all it says. So how do the priests know that that means you can't have out of wedlock and stuff like that? Do they just make it up so people can have no fun, or did God come down and say the rules super specifically. I'm not trying to be sacreligious or anything, but where do they get so much detail from?