What Springcurl and RCW said.
My mom started when I was quite young, and gradually covered a variety of topics.
Also, my friend had a book geared towards pre-teens when we were about that age. I asked my mom if I could borrow her book. My mom said it was fine and to just come to her if I had any questions. It helped alleviate some of the embarrassment I was feeling about having these questions/desires/interests.
Did I mention my mom is the best?