What do I do when I get my first period?

I know I am getting close to the age of having my period and I need some help. What do I do if I get it in cl and it gets all over the place? I would be mortified. I am scared. My friend just got hers in the middle of gym but she was lucky and nobody saw. How would I tell my teachers? I am to embarred to ask my mom. So please help!