Count your first day of full flow as DAY ONE. When you get to day 12, start trying. Most people ovulate around day 14. With your age though, after a few months I would get a little help from a fertility clinic.
Ovulation predictor kits help with pinpointing the best time. Also, try to be pro-active and talk to your ob about running some basic tests to check whether your tubes are open, whether you ovulate, etc. Don't wait to ask for help, I wish i did not wait as long as I did.
Good luck!
thats how i did it 3 times, im 40 and on my third pregnancy and it works everytime, start taking your prenatals NOW , and folic supplement, get your body ready before getting preggo .
the first month is a test month , just see when you are ovulation ( buy an ovilation kit ) , if you ovulae 10 days after the first day of your period, then on your next period the next month , around the 10 th day , start having sex , also check your ovulation kit, you can start having sex the day that you get " a happy face" meaning you are ovulating :))) i always got pregnant if i did it that way !
good luck in getting pregnant