Weird question, but here it goes

How do I know when I'm fertile? I'm trying my best to avoid pregnancy, and I wanna know what days to avoid having sex. My periods are quite irregular (always have been, but its worse now because of a medication I was on that made me miss a complete month, so now idk when my period is due), so how do I know? Also, is it best for me to avoid sex while ovulating, or during my fertile days?