It's different for everyone, of course. One of my friends didn't get hers until she was sixteen (which is relatively extreme.)
Really it'll come when your body is ready for it. And you'll definitely know when you get it. I'd say around thirteen or fourteen is normal, but it's different for everyone. Don't worry if it's a little bit early/late. If you do get worried, don't hesitate to talk to your mum or a doctor. It may seem embarrassing at first, but it's better than worrying all of the time.