ANDY'S ANSWER: You could still be a year or two away from getting your first period, but it never hurts to be prepared. Make sure you are always carrying a pad or a pantiliner in your purse or in your locker at school so you can put it on as soon as your period starts.
It's not like you will suddenly start gushing blood. One time when you go to the bathroom you might notice some red or brown stains on your underwear. That probably will be your period starting and that's the time to start wearing the pad or pantiliner. From then on, you will have to be dilligent about making sure you always have something on hand for when your period comes.
If you have your own money, you have the option of buying a box for yourself, but it's probably more practical to talk to your mother and ask her where she keeps her supplies and if you can access them when you need them.