I assume by "learn harmony," you mean "learn how to harmonize." If your question is deeper than that—if you want a full understanding of WHY the following comments work, I am one of a host of educators who would be happy to teach you a collegiate-level course in music theory. For a fee, of course. read more
I think every person has their own best and fastest way to learn. It may be with a teacher, online courses, reading books, or whatever. You just need to try and discover your own way. read more
Learn a song's harmony part by rote, and sing that alongside someone else singing the melody (or a recording of yourself). The simplest harmony part is a third harmony -- that is, whatever the melody part is, you sing a third higher, within the major or minor scale of the key signature. read more