Talk to your mom and see when hers started. Remember, every girl is different so you can't base this on your friends. You can get a general timeline from any sisters you might have and your mom.
A good sign you're getting close to the time when your first period will arrive is if you notice a discharge coming from your vagina. It might be thin and slightly sticky or thick and gooey, and can be clear to white or off-white in color. Usually, this happens about 6 months before you get your first period.