A teacher is best, or at the very least a friend that you learn it with. Learning Music Theory is utterly useless if you do not do ear training, and to do that well you need someone to work on it with you. Teacher, hands down.
I've taken the AP and it would be amazingly futile to try to learn something like that by yourself.
Also, when you're listening to music, grab some staff paper and do some melodic dictation. Analyze form, do some chord analysis. Music theory isn't somthing you just learn, it's something that you really have to train and work on constantly.