I learned - and am still learning - from a variety of sources. I took electronics shop in high school which was valuable, and I learned by getting an amateur radio license, too. A couple of influential people have been my late uncle, a Bell Labs guy and serious ham and a friend who was a nuclear electrician in the Navy. I also picked up a bit when I started restoring old radios and reading the books about radio.
Since, I've found great material at Pete Millett's site, the books by Morgan Jones and Bruce Rozenblit, AudioXpress and Sound Practices magazines, the TubeCAD Journal, the Boozhound Labs site is terrific, and I've Googled around lots of other hobbyist sites on the Internets.
My advice is to go hands-on. Pick a project. If you want solid state, build a CMoy. Buy the parts a few tools and just jump in. It is nicely documented and there are hundreds of people here who can answer your questions. You'll pick it up as you go along. If you're interested in tubes, go find an old five tube AM radio. You should be able to get one for $20-$40, and restore it. If you stay away from boutique parts, you can gut and restore one for about $30-$40. You'll learn a lot going through the radio and the bonus will be great AM sound - you'll be surprised at how good one sounds.