I took a lesson after 2 years of boarding, and although I didn't learn anything monumental from the lesson, I did learn a number of things that I wasn't doing right. I spent quality time correcting those deficiencies and it has made a large difference to my riding.
My advice would be that you take a lesson when you start, and at least one lesson per year for the next couple of years, at the beginning of each season.
I told my kids, "Someday, you'll have kids of your own." One of them said, "So will you."
-- Rodney Dangerfield.