3rd month TTC

My husband and I already have two sweet boys (they are from my previous relationship and he's adopting them). We want a baby but we are already frustrated. We've been trying for a few months and we are going stir crazy. Everyone keeps telling me that you ovulate on day so and so if your cycle but I have no idea how to determine what the first day of my cycle is. Is it the first day of my period? The first day after my period? How do I know? How do I know if I'm ovulating? This is so much more confusing than I thought it would be and any help and information would be amazing!