Congratulations! You are on your way to becoming a woman.
Girls can start their periods anywhere from 11-16 years old. A lot of ‘when you start’ depends on your genes so ask your mum when she started hers. Also, girls with a high body mass index can start a bit earlier. Some research suggests that the menarche (first menstrual period) is gradually getting earlier over generations - maybe due to a few things like improved diet and better health. Rest assured that your friends will soon be following you and you can be there to give them advice when they do.