When should I start doing ovulation test?? Plz reply..

It's been two years that we r trying to get a baby but it is not happening...we also visited doctor , where we got the response as a healthy couple..I did ovulation test even through ultrasound..report says m ovulating but the days r not same ...like in one month it's Falling on the 17th day and the other month on 15th day. However, our meeting quite alternative..So, my question is from which date should I start testing myself? Plz suggest what r the other measures I should try to improve my fertility? Is there anything wrong in me?