The first day of your period is day 1 in your cycle.  In the typical 28 day cycle, ovulation occurs around day 14.

But a lot of women don't have that perfect 28 day cycle.  So counting days might not work for them.  Instead they can check their temperature, watch their discharge (your discharge should look like egg whites around that time), or get an ovulation monitor.  

If you are trying to get pregnant: If you think you ovulate on day 14, you'll want to have sex around days 10-15.  You can use www.mymonthlycycles.com to help you keep track of your cycle and plan when you are most likely going to be at peak fertility.  Make sure you are seeing an ob-gyn, taking prenatal vitamins, and doing some research (look up the delivery process, prenatal development, breastfeeding, etc.)

If you are not trying to get pregnant: Be aware that while there are days when you are more or less fertile, there are no days when you are completely immune to pregnancy.  So assume you are always fertile and use protection every single time.  You can get condoms at the store or you can talk to an ob-gyn about hormonal methods.  Avoiding sex when you are near ovulation can be a good backup method when used in addition to condoms, but it has a fairly high failure rate for a lot of couples who try to use it as a main method because it can be a bit tricky to get the hang of and it requires a lot of self control.

If you have questions or comments, feel free to head to the forum.