So you’ve been playing guitar for a couple years now, and you’ve got all the basic techniques down. Bar chords are no problem, you can solo with the major and minor pentatonic and blues scales, and maybe you’ve even gotten into learning some 7th chords or rootless voicings. Now the question becomes: do you know what you’re playing and why you’re playing it? This is where music theory comes into the equation. If you want to better understand chord makeups, chord progressions, and what scales and melodies can work over them, then you have to start to understand at least the basics of music theory. Applying what you learn from theory can open up new worlds of possibility in your playing and composing, and can really help spur on some periods of creativity and inventiveness. However, tackling these concepts immediately on the guitar can prove a daunting task because of how string instruments are constructed, so I recommend learning some basic piano.
But I’m a guitar player you say! Why should I learn piano? Well, piano really is the ultimate theory and composition instrument because of how logically it’s laid out for you. (Disclaimer: If you haven’t done so already, you’re going to want to learn how to read notes on the guitar so that translating what you learn on the piano to the guitar won’t be so difficult. I personally recommend starting with Mel Bay’s Modern Guitar Method: Grade 1, and then moving onto Berklee Press: A Modern Method for Guitar, Volume 1.) Even getting through just the first half of one of these books with a good teacher should at least give you the foundation to figure out how to translate notes from the piano to the guitar. For example, you might see something as follows when you first begin learning theory, and if you don’t know the notes on the treble clef it’s almost impossible to understand. It explains the order of half steps and whole steps which constitutes the major scale in any key.