Girls usually start periods between 11-16 years. Each girl is different so some start early/late. Hormones decide when periods start, you have no control over it. Periods starting is a sign that you can now get pregnant. A girl can actually get pregnant even before her first period. Starting periods is not a sign that you need to have sex-wait until you are ready. When you become sexually active use a family planning method to prevent unwanted pregnancy, HIV and Sexually Transmitted Infections (STIs).
Attribution: Straight Talk Foundation