It really depends on more than just when your last period was. You'd atleast need to know when your last period was before that one and count about 14 days from the time of your last period up to the time of your period before that. Then you can know what day in your cycle you typically ovulate. Also, women don't always ovulate every cycle if there are complications. Although typical cycles are 28 days, many women have cycles ranging from 22 days up until anywhere near 45.
I would suggest downloading the free trial of ovusoft's fertility software from downloads.com or something. Google it. It will help you determine your fertile days and when you ovulate, and it can be really accurate if you input your morning temperatures in.
Good luck hun... and just so you know, if you have a 28 day cycle, you should ovulate around November 10th. I'd suggest having intercourse every day (or every other day if you've been trying for a while and think you're partners sperm count may be low) from now up until november 15th. Best of luck!