How do I tell my mom when I've had my first period?

As you are only 10 you shouldn't be discussing sexual health on here. But my advice would be talk to your mom now and ask her advice. She was 10 once. She has had her first period! A mother is best suited to talk to her daughter in advance to help her daughter cope with all the aspects of her periods, and especially preparation for her first one. (I am the father of daughters). There is no need to be scared. Talk to mom.