If you want to learn music theory, one of the best instruments to take up is the piano - or even just an electronic keyboard. The piano is great because you can see all of the notes in front of you and learn how to use two hands at once, playing independent notes. You can learn all types of ways of playing the piano, as well, including classical style and jazz style. But is there another way to learn music theory if you don't want to become a piano player right away?
Absolutely: you can buy yourself an inexpensive acoustic guitar and learn all about scales, chord progressions, and the relationships between notes. If you already own an acoustic guitar but don't know how to play it, and don't really understand music theory, this article will help you get off on the right foot. Or, in this case, on the right finger.
How to Learn the Basics
One of the first things you'll want to do is actually buy a metronome. A metronome is that annoying little device that you set in order to hear a constant tempo. Why do you want to play with one right away? Can't you learn rhythm and tempo independent of learning other music theory? Well, you can, but practicing with a metronome right away is vitally important because you might not realize how far off your internal sense of rhythm and timing is. You can play the metronome slowly at first, in order to make things more accommodating for you as a beginner. Once that gets easy, you'll know you're ready to kick the tempo up a notch and practice playing your music with a more difficult and faster beat.
You'll also want to focus on basic music theory you'll find in any beginner's music book: learn about major and minor scales. Be sure to pay special attention to the relationship between notes in these scales, because once you understand how these relationships work, you'll be able to essentially understand these scales no matter what the "root" note is. For example, playing a "C major" scale on guitar should be just as easy as playing a "G major" scale. On the piano, this might be a little trickier because of the difference between white and black notes, but on guitar, you know how easy it is to move up a half and whole step.
Putting Together Chords
Once you have the basics down, you can move on to ideas like chord progressions. The I-IV-V chord progression and its variables are one of the best ways to get started: you can play a variety of songs out of that chord progression, and even improvise your own melodies quite easily. When you start to see the relationships between chords, you'll get a better understanding of how a song is constructed, and how one chord might be used in order to "set up" the next. There are a lot of different elements that go into constructing a song, but once you know how to play basic chord progressions on guitar, you'll feel like you've really improved a lot.
Strumming and Picking
Strumming and picking are essential skills on the guitar, and you'll want to learn how to use both of them. If you memorize a specific finger-picking pattern, for example, you will be able to produce a sound out of your guitar that sounds like two people are playing: it's a great way to sound more "advanced." Strumming the guitar is an essential skill, as well: if you can strum a guitar with clarity and authority, you can really start to play.
There are a lot of other elements at play when you learn music with the guitar, but these basics should hopefully give you an idea of what to expect.