Let's take a trip into your panties, for a moment. You've probably already noticed that you get stuff in your underwear and thought "eww" or "didn't I wipe myself?"
That stuff is called mucus, and it tells you where you are in your cycle. To put it another way, the mucus can help you predict when you will get your next period. Of course, if you haven't had a period yet, the mucus in your panties just tells you that you will probably get one soon. Yay!
In adult women, bleeding usually lasts 3 - 7 days. After that, the vagina is kind of dry. Sticky mucus will show up in your underwear. As the days go on, it gets clearer and stringier. For a couple of days, it's slippery on the toilet paper when you wipe. This is called "fertile mucus" and is the signal that your period is coming in about two weeks.
Then the mucus gets sticky again and a few days later, you get your period. (Oh, and that's what we mean by "cycle" by the way: something that keeps going around and around). Some girls and women have a very regular cycle; others never do. There is no "normal" cycle.
So what about you? Just like there is no "normal" cycle, there is no "normal" age to get your period. Puberty can start as early as 7 years old or as late as 15. If you already have breasts that kind of look like breasts, hair on your vulva and mucus in your underpants, you'll probably get your period within a year. Yay!