I don't know if I can get pregnant:(

I'm 19 years old and I've Ben trying to get pregnant for 2 years now and I got a pap smear done a while ago and my doctor said everything is ok.im VERRY irregular on my periods so I don't know when I ovulate or when I'm fertile and when it's the best time to have sex to get pregnant.sometime my period will skip 3 months .I don't know what to do! I've Ben trying for so long and haven't gotten pregnant.what do I do?