Hey, all I can say is that there is nothing to worry about! You'll probably feel a small twinge or cramp before your period comes. I was 13 when I had my first period and I always know when I'm going to have mine because I have a stomach ache a few days before and I feel slightly sensitive and tender 'down there' :L . Also you might feel slightly emotional! It's just your hormones, you'll probably find yourself getting really angry about the slightest thing, or wanting to cry about anything, but don't worry it passes soon enough.
Most girls get their first period in their early teens, but some don't get theirs until 16 or later. Don't be scared, you could have a few more years to go yet, and if you do a lot of exercise that might put it off for a bit. All i can say is make sure you carry sanitary towels around in your bag at school, so that if it does come you are prepared. I know that I had quite a lot of discharge (that gooey white stuff that you might get in your knickers sometimes) about 6 months before my first period.
When it does come you'll soon get used to it, I'm 15 and mines still not regular but it'll settle down after a few years. I hope this has put your mind at rest a little bit! Don't sit waiting for it to happen because when it does it is a bit of an inconvenience, but us girls have all the tough stuff to deal with, and it soon becomes just a normal thing, just part of growing up (: