This might be useful:
http://www.fpa.org.uk/Information/Readourinformationbooklets/bodyworks
Scroll down until you get to the section on the menstrual cycle. Most women's cycles are 28 days, with the first day of your period being day one, and with ovulation around mid-cycle. As an example, my natural cycles are exactly 28 days, I bleed days 1-7, then again 21 days later (I used to be able to time the start of my period to the minute lol); ovulation would be around days 12-18.
It can take 6-12 months to settle into a regular cycle after coming off the pill do so do be aware of that. It might help to keep a diary if you're a little confused about where you are in your cycle, and that will also allow you to see a pattern emerging so it's easier to predict.
Hope that helps x