Hi there. Remember that girls your age are often irregular and it is completely normal. Your period may be all over the place for when it shows up and it is just your hormones getting regulated over time. Important to use good effective birth control if you are sexually active because when we are irregular, it's hard to know when we might ovulate. You can use the app here at med help (my cycles)-- go to the top of this page and look under devices and apps, pull down the menu and you should see my cycles) to track your period and you'll start to get a sense of your cycle. Day one of your period is cycle day one. Most women have a 28 day cycle but again, you are in your teens and may be irregular. Just carry a tampon or small pad with you in case it starts. good luck. And this is all perfectly normal and we all go through it. :>)