You should ask at some forums where members speak English, because many of people here don't.
And... what is the problem?
Type "how to tell my mom i got my period" into Google and then you can find good advice- like this one : Oh come on. Make your mother happy and tell her. Yes, I said "happy". Most women consider it a milestone when their daughters start their period. You'll probably get a hug, or some ice cream or something.
The only way she would NOT be happy is if you used up all her pads, and she found an empty box on the day that SHE needed one.
Look, toilet paper is not going to cut it for long. I don't care how light your periods are right now, that can change at any time. If you have an accident somewhere away from home, that will be a lot more embarassing than having to spit out the words "Mom, I got my period".
Your mom's already been through this, and nobody can help you like she can.
But if you just can't tell her out loud, write a note and put it in her wallet. She'll be sure to see it there before long, and no one else will find it. (From http://answers.yahoo.com/question/index?qid=20080120224004AAjBcfd)
A chválím tě za snahu se učit anglicky. Koneckonců - i tohle je způsob, jak si zlepšit znalost jazyka.
Thank you all for the great advice, it helped me a lot