That depends on the person learning it and the depth at which you wish to learn it. Some people learn by hearing a lecture, seeing things demonstrated, or reading a book on the subject. Music theory is learned by three year olds as they understand that a whole-note receives four values and a half-note receives two, or you could go in deep enough to discuss the ten principle areas of music and contrast those in lieu of traditional music and modern music.
If it is recreational, meaning music is a hobby or side interest to you, I would suggest asking specific questions (ex. "What does C/G mean?" or "How do you get between two keys smoothly?") or reading books directly related to your interest. (guitar, piano, singing, etc.)
If learning music is something directed from a passion or professional goal in your life, I would suggest taking a class at a community college (if possible to non-majoring applicants), pursuing a degree in music (learning theory is required), or seeking an educated musician to tutor you in the subject.
A good place to start learning about the basics of music theory would be Ralph Turek's The Elements of Music theory book and workbook.