Wait, what is music theory? I get this very question from a lot of folks. The phrase 'music theory' isn't exactly self-explanatory. Let's see if I can give it an understandable definition. When you study or practice music theory, you are breaking down music into it's individual elements, defining them, fitting them together and seeing how they work. Pitch, melody, chords, notation, rhythm, notes, these are all different elements of music theory. By understanding every aspect of music and how all the pieces fit together, you can easily figure out how to make melodies and songs that will move the listener in the exact way you intended.
Why is that some songs sound sad? Or triumphant? Mysterious? How is that a song can build tension and release it? If you learn music theory, you will understand how all of that works, and you'll know how to do it yourself when you write your own songs. Sure, some people can get away with not learning any of it formally. They have managed to figure it out intuitively. I'm not one of those people. For the most part, I can't listen to a chord and be able to guess what notes it's made out of, for example. At least, not yet. This is why I learn theory.
After you've gotten started playing your instrument of choice for a while, you've got a good base for learning theory, and you probably already know a lot of it already and don't even realize it yet. I don't recommend studying theory before you've played any music.
The first thing you should learn is how to read sheet music. This doesn't necessarily have to come first, but it will make your life ten times easier, by my extensive calculations. Though you could, theoretically, learn music theory without knowing how to read music on a staff, I don't know of anyone who has. Theory is usually expressed using the staff. Learn what the staff is, how to read notes and rhythms and by the end of it you should be able to sight-read at least some really simple tunes. If you want to be a composer, then you should learn some piano too. I've covered that in a couple of other articles. By taking up piano you can get the triple-benefit of piano-playing skills, the ability to read sheet music, and some basic music theory all at once.
Now we get to the meat of this. While the Makeshift Musician usually recommends that you learn things on your own, perhaps in this case it might be good to get a lesson or two from someone else. Music theory is both complicated and abstract, like math or a language, so it can be difficult to learn without someone there to make things clear for you. You don't have to get a Bachelor's in Theory or anything, maybe just a couple of classes to get you started, or find a mentor to help you out. Check local colleges and adult education programs and see if they offer some sort of basic music theory course.
If, on the other hand, you're like me and want to slog through it all by yourself, then I have a great book to recommend. In fact, I recommend it even if you are getting lessons:
Edly's Music Theory for Practical People
This books starts from the basics and goes all the way through up to the most complicated chords and unusual concepts. It is taught in a clear, linear way (don't you hate it when books tell you to skip ahead and back just to get everything?) It's written in a conversational style so as to not be confusing and it has a goofy sense of humor, which I like. If you read this book while you're learning an instrument, it'll all come together pretty intuitively.
There are several other books that will teach you music theory, this is just the one that I've been using and I like it a lot. If you have any questions about learning theory just shoot me an email: email@example.com.