how do i know when im ovulating?

my periods are irregular, so i dont know if what this chart is telling me is right or not...so how do i know when im ovulating? i think my period is going to start either tomorrow or sometime this weekend or maybe even tonight because im cramping a little bit...im on the pill, but friday i will be out and we are TTC...can anyone help me and tell me how many days after your period is when you start ovulating and also if it is possible to get pregnant while on your period too? tests are too expensive and dont have anyone to take me to the dr for O test...please help