How To Calculate Ovulation Day?

pregnancy/period
hi my name is kyley and I have a question im trying to have a baby but I don't know when im fertile or ovulating, on december 2nd 2011 I started my period and it lasted about 4 days it was normal on my standards but then on december 20th 2011 I started bleed again and it also lasted about 4 days the only thing was it wasn't like a normal period that I usually have this time it was very light kind of like spotting, im trying to conceive and have a baby but I don't know when im ovulating or fertile, can you please give me some information and help me figure out when I should start trying for a baby. I been trying about two months now and still no baby.