You should be having sex on approximatly days 13-15 if you are on a normal 28 day cycle. (If you dont know day one would be the first day of your last period, so may 29th). You might also consider buying an ovulation kit from Wal mart or something. It will cost just under $20 and it will basically tell you when you're most fertile days are. that would be my best recommendation for you.
Another thing though, if you have just gotten off of your birth control it coul take a while for those chemicals to fully leave your body, just so you are aware. 9and If you were'nt just on BC then ignore that last part.)
Good Luck!
you need to have sex during your fertile days. you can always use one of the online ovulation calculators and see when you are most fertile:) you should also be taking a vitamin with folic acid daily.