About two years in college on a 49cc Yamahopper scoot. Just rode it.
Fast forward four years. Friend of a friend commuter was moving up to a Yamaha something or other. Dealer offered him $500 trade-in on his 79 KZ750. Said he'd sell it to me for that. I'd "always wanted a motorcycle" so took him up on it.
Went to his house, gave him a $500 check. Had him explain the controls to me. I'd ridden MCs once or twice before and knew how to drive a clutch before so, with the scoot experience, it wasn't totally foreign to me. Hopped on the bike, got on the freeway and rode it about 10 miles home. I don't remember it being particularly scary or anything but my shifting probably wasn't that smooth.
Got home and got off it to put the side stand down. (Habit from doing that to put the scoot on the center stand.) It promptly fell over on the left side. Me: "Damn. these things sure are HEAVY."
Got a learners permit that requires you to ride with someone else but didn't know anybody who rode so just rode it illegally for a few weeks. Practiced some slow stuff in the Park-n-Ride across the street and riding around the hood.
Went to take license test. The thing had some miles on it from being a multi-year commuter and didn't idle so well. Stalled during the test, examiner got pissed at me for wasting his time. Told me to get it fixed before I came back.
Didn't do anything to it. Practiced some more slow test type stuff in the Park-n-Ride. Went back a couple of weeks later. Different examiner. Stalled on me several times so I had to put my foot down to keep it upright. Examiner: "Well, technically I should fail you but it looks like you know what you're doing so I'm going to pass you."
I had the basics down, never took any training (didn't know it existed) and just learned from riding. For the first 25k, as they say, I did learn some things seat of the pants but essentially rode "the first 1,000 miles 25 times" so not a heck of a lot of skill improvement.
At that point I got around to reading Proficient Motorcycling. I learned a couple of things but most of it I'd already figured out for myself. (I knew what countersteering was long before I even rode the scoot.)
Around 2004 or 2005, I started watching YouTube videos and finding stuff on The Internet and devoted myself to trying lots of different things and working on my riding skills. Started employing a mentality that had worked very well for me to improve my downhill skiing - every turn is a practice turn that you can learn something from. Made a HUGE difference in my riding skills and made riding a lot more fun.
I am certainly nowhere near being a totally awesome rider but these days I am confident that I can "ride my way out of a wet paper bag."