Help please!

So I guess I just didn't pay all that much attention in 6th grade health class. But can someone please help me and tell me sorta in detail how periods work. I know it sounds stupid.  I know the basics but I would like to understand my body more and be more sure. I live with my boyfriend and we have sex quite regularly. But we are NOT ready for children. While replying to this post could you also answer a few questions for me?