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.
Yeah, that's me, Tracer Bullet. I've got eight slugs in me. One's lead,
the rest bourbon. The drink packs a wallop, and I pack a revolver. I'm
a private eye.