I hired a tutor. Even with weekly lessons for 2+ years, it required a lot of sustained effort and dedication. Like most everything in music, it requires a lot of practice and there are no real shortcuts.
Nonetheless, you can definitely teach yourself the basics via books and the websites recommended below. But once you want to go past the basics, you're going to want some kind of expert/teacher/tutor to ask questions of and to review composition exercises, so that you can understand harmony, voice leading, etc.
That being said, if you're a musician, learning music theory is enormously helpful. You understand what you're seeing on the printed page as much more than a series of notes and chords, and your musical intelligence as you listen is greatly increased. Good luck!
Nonetheless, you can definitely teach yourself the basics via books and the websites recommended below. But once you want to go past the basics, you're going to want some kind of expert/teacher/tutor to ask questions of and to review composition exercises, so that you can understand harmony, voice leading, etc.
That being said, if you're a musician, learning music theory is enormously helpful. You understand what you're seeing on the printed page as much more than a series of notes and chords, and your musical intelligence as you listen is greatly increased. Good luck!