Are you looking to learn music from foundations including reading or just to be able to harmonize with people singing along to something?
Just the latter (but also with people singing a cappella). I'm comfy with singing, and with the gist of following notes on a page.
You can get a very long way learning by doing if you're musically inclined. A really long way. Apart from courses etc. I would strongly reccommend joining a largish choral group/choristers/choir if available. That and picking out the harmonies in music on the radio if you can (absent other people around listening!)
I think it's harder as one gets older; I was lucky enough to start to learn by imitating my father in church, some time in very early primary school. Helps that very old hymns usually have very straight forward chord progressions (by necessity, because they were designed to be sung in churches that were designed to be echo, prior to microphones being a thing. So the chords can't be too 'jazzy' if the echo smashes them into one another)
I don't know if I'm musically inclined, and I'm getting older, but I'm confident anyway :) Right now I can sometimes grab onto a harmony for a very short phrase, but then I lose it again. I feel like I have some "sense" of music, and people have told me I sing well. I need to get out of the shyness, though, for sure. This will be the "make or break" for me. I'm the same with languages -I can learn them, but for the longest time will be terrified -absolutely paralyzed- to use them. At some point there will be a language emergency and I'll naturally out myself, at which point everyone around me is shocked at my Secret Agent ability, ha ha. Maybe I can create a singing emergency??
Where I am there are lots of fabulous singers, and lots of harmonizing, so I certainly have great people to observe and imitate. Choirs have long been a no-go because of parenting (kid can't function much in the ensuing hours if I keep him up/out in the evenings and I have no child care). I did get to go to a 10 hour workshop a couple of years ago that I couldn't get enough of -world music in four parts a cappella! I'm hoping I might have daytime child care by summer, at which point I could participate in workshops. So, strategies until then.
Here are a few I love:
Wailin' Jennys (Bird Song; One Voice; etc)
Lennon & Maisy
https://www.youtube.com/watch?v=PGca52zSLWE ;
https://www.youtube.com/watch?v=n-lLlEtoAS8Right now I give it a shot while listening to Billy Bragg (especially where he has Natalie Merchant on harmonies), Ingrid Michaelson, etc.
