My first game was a JV game and I worked the U. My training was mostly rules and very little mechanics and I didn't know any position any better than another.
On the 4th play, the HL came in to let me know part of my job was to spot the ball.
You learn through experience!
I worked several positions that first year in sub-varsity and youth games and started at HL the following year when I joined a varisty crew. I took that position only because it was the opening the crew needed to fill. I think you can argue for just about any position (besides R).