I get my period every 19 to 20 days and I can't find when my most firtile days are.

Between the 7and8day

A good starting place is about 12-16 days before your next period would be due to start. While cycle lengths may vary, many women will keep a fairly regular number of days between ovulation and menstruation regardless of length of cycle.
This gives you only the average, however, and plenty of women will ovuate sooner or later on a regular basis.
Watch for possible signs that your body is preparing for ovulation... maybe a bit of cramping, increased interested in sex, cervical discharge that is more fluid. For some women it can be difficult to tell and sometimes one month may be very different to the next.
Ovulation predictor kits work for some women as does temping and charting. Both of these can be frustrating for other women however.
Another way some couples find helps is to simply have sex often... every other day for example, with maybe adding a few more times when you think you may be the most fertile for good measure.