Some girls get their first period earlier than this, and some get it much later. If you are worried about either, talk to your health care provider. Yours may be a few days shorter or longer, and for young women especially, the length of the cycle can fluctuate a lot. Basically, when you bleed, your body is shedding the lining that it built up during the last cycle. This is ovulation.
Some women actually feel at their best during ovulation.
It's different for different people. If the egg your body released doesn't get fertilized, hormone levels drop. You may experience pre-menstrual symptoms during this time. About 28 days after your last period, you start to bleed, and the cycle begins all over again. If you find that your cycle is fairly regular, it can help you predict when you'll get your period.