Use mymonthlycycles.com. Enter your past period dates and use the menstrual calculator to give you an approximate time. Of course it can't take into account your exact levels of stress, exercise, etc., but it does give something.
*If you are talking about your first period, then I agree with the previous post. That will happen whenever your body is ready.
No because no one can deciede when you will have your first period, only you body can decide that, if you find some way to talk to your body and it answers, please tell us, but until then, your time will come, dont worry
if u mean u have had your period but want to know when your going to start or finish again try these http://www.beinggirl.co.uk/your-body/period-predictor/ this 1 tell you about what happens http://www.beinggirl.co.uk/your-body/periods-calendar/