When I was in 5th grade, our school introduced this topic to us. I'm not sure what grade puberty is introduced now...because the average age of girls getting their period is decreasing!!! I think you should be open and comfortable with your daughter. Explain that her body will soon be going through changes soon, and having a period is one of those changes. Just cover the basics....a book might help. You are doing the right thing by wanting to talk to your daughter. By being open she will know that she can come to you with any further questions!