When you have your first periodYou will know when you will start your first period because you will see a whitish discharge in your underwear a month or 2 before.
The first period is always the hardest. Carry a pad at your size around because your school might not have your size. I wasn't expecting mine and the school gave me MASSIVE ones and trust me, thats not nice! I had my first period (this sounds a bit wussy, but I'm really neurotic) and because when they showed me how to put it on, it was bloody the pad i put on before i knew how, and i was freaked out (no i didn't scream in the toilets). In fact, I was scared to go to the toilet for that day!
If you do have your first period, don't hesitate to write something on my wall and i'll be happy to help. Also (like me), it doesn't help if you have PE the day after your first period, you have a bleep test (a running test to test your stamina) and you end up hyperventilating after it.
Read on for more info!