When you're a teen, periods can be irregular sometimes. You can keep a calendar where you mark when you start your period and what symptoms you have beforehand. Also mark when it ends. Do this every month and you should start to see a pattern. You might notice that you get a cramp or two the day before your period starts, or maybe you notice that your pants are tighter because you're a little bloated a day or two before. I always keep one tampon in my purse just in case. There is also a menstrual tracker on this website that you can use for free.
Ovulating usually occurs halfway through your cycle, approximately two weeks after your period ends. Many women find that they produce a clear discharge when they are ovulating, but not everyone.
Hope this helps! Let me know if you have any other questions!