Learning harmony is all about developing a sense for what an added voice can do to make a song feel better, while not being the main voice.
The only way to learn harmony is to try singing harmony. Turn on a song, hear the melody, and then sing something else over the top of it. Does it sound good? If it’s your first time you’ll probably stumble upon some notes that work and some that don’t. Perfect. Keep going.
And you won’t know the theory behind the notes that seem to work but that will come later and isn’t as important anyway.
