When to start prenatal vitamins?

SS
My husband and I are ready for a baby, so I went to the doctor and did my pre-conception consultation. I was told that it's the best that I start the prenatal vitamins months before getting pregnant. How should I interpret this? 
I am going to start taking the vitamins soon, but we also want to start trying this month. Should we wait for next month because I haven't taken vitamins regularly? How big of a deal is this?
Thank you!