Are You There God, It's Me, Margaret by Judy Blume

Margaret Simon, almost twelve, has just moved from New York City to the suburbs, and she's anxious to fit in with her new friends. When she's asked to join a secret club she jumps at the chance. But when the girls start talking about boys, bras, and getting their first periods, Margaret starts to wonder if she's normal. There are some things about growing up that are hard for her to talk about, even with her friends. Lucky for Margaret, she's got someone else to confide in . . . someone who always listens.