Hey Willow -
Congratulations on becoming a woman! Okay, so when you first get your period, certain hormones have begun producing in your body; in other words, your ovaries have kicked in, and you will begin ovulating - which happens two weeks or so before your period (that is the time when pregnancy can most likely occur) and two weeks later, when your body notices you aren't pregnant, you'll get your period for about five to eight days or so.
But, when you first start, your body hasn't really gotten its rhythem yet. So you could not get another period for a little while, maybe skip a whole month or two. But eventually, you'll be on track and if you're lucky you'll be able to figure out exactly when you get it. I get mine about every 26 days, most girls and women get it between every 28 to 35 days. Everyone's different!
Good luck with the whole thing, and if you're nervous about getting it in school or something, just wear a pantyliner till it comes so you don't get caught unprepared.