Puberty and periods are controlled by hormones in your body, especially sex hormones called oestrogen and testosterone. Bascially your body is getting ready to become adult, and all sorts of physical and emotional changes happen. Testosterone causes boys voices to deepen, they grow hair, and they make more muscle. Oestrogen causes girls breasts and hips to grow, and a whole load of other hormones starts their menstrual cycles. There are lots more details – ask your biology teacher or your parents for more information.