I'd say I learned how to repair electronics, by NOT learning how to play baseball, etc, while I was growing up
I like the challenge of fixing obsolete equipment, it's fun because it forces you to be resourceful.
I would suggest some books about radio/electronics theory from the 50s and 60s, targeted at the vocational market. Temper that with buying old junk (like AA5 radios, or an old guitar amp, or whatever you're into but nothing too complex at first), and working on it, so that you don't get bored by pure theory. Plan to devote at least a workbench, and probably a room to the hobby, stuff seems to multiply on its own after people learn you're interested.