Hi ,
Best thing to start doing is just take note of the length of your cycle for a couple of months. Day one is the first day of bleeding (usually only counted if you start bleeding during the day not late afternoon). Record the length of the cycles then average them after a couple of months. The second half of your cycle (after ovulation) is called the luteal phase and is generally 14 days. So if you average a 27 day cycle you should in theory ovulate on day 13.

All this aside my advice would be have sex every second day from day 10 to day 20 of your cycle every month and you will get pregnant eventually if it is going to happen. If you don't have a regular cycle at all it is a bit harder to be very accurate.

hope any of this helps. good luck