you take the day that your period starts as the first day of your cycle, so if you have a 28 day cycle you would usually ovulate anytime from around day 10-16, you can go on line and fill in the particulars for your usual cycle and it will come up with approximate
ovulation dates for you just google
ovulation calculator, another more precise way is to keep and eye on your cervical muccus, egg white and very stretchy is fertile, you can also buy ovulation strips and start testing from day 10 in