You may have Googled “how to use music theory on guitar” or even “why learn music theory.” There are so many moving parts in music that can cloud our music-making. So, here is a good mindset that will give you clarity on the subject.
Let’s look at music theory from two perspectives – as a TOOL and as an EXPLANATION. You should be able to change the gears in your mind back and forth between the how (composition – tool) and the why (analysis – explanation). Let’s take a look at explanation first.
I am all for blind experimentation in music – picking up your guitar and just going for it. The issue is this – can you use and re-use the fruits of that experiementation? Let’s say I find a great lick during a free-improvisation session in 12th position. Can I re-use this sound during other songs or jam sessions? The only way to do that is to be able to EXLPAIN what scale the lick uses, what key/mode the chord progression was in, what chromatic tones were used, the spacing of the notes/intervals, etc. The analysis could be a thousand different things, but the point is that I can use it again when I want to get a similar sound. So if the lick was in E minor and incorporated the 9th (F#), I can re-use the idea in another song in, say, G minor by targeting an A-note (the 9th). If I am stuck using the 9th only in one position because I don’t know what is going on, then I am seriously limited in my creative ability.
Another scenario – my friend plays a chord progression he wrote and I want to solo over it. The only way I can effectively solo – to really get the most out of the progression – I need to know what key the progression is, what notes are in the chords, if any chords are out of the key, etc. That gives me all I need to know to select scales and notes to use and then just go for it. In summary, the analysis of music gives the musician the ability to label, transpose, re-use, modify, and basically play around with musical ideas in a variety of ways that would not otherwise be available to them.
Music theory can also be a TOOL – but you need to be careful that you do not make it the only tool. Whereas songs/solos made with little-to-no theory knowledge sound repetitive, lack momentum, and sometimes irrational, songs/solos that are written with strict theory guidelines can sound boring, predictable, and lifeless. Guitarists who do not know theory tend to copy what they have learned (other people’s songs) and have trouble generating original ideas, and guitarists who rely on theory too much have no influence at all except rational/mathematical musical structures. When you learn a theory concept, the only way to make it meaningful is to try to use it creatively in your own playing. If there is no application of the concept, it is completely meaningless. You must be able to take a raw theory concept and use it as a tool to generate new music.
For example, if I write a riff in E minor but it needs a tweak, I can try it in E Dorian or E Phrygian or E Phrygian-Dominant or or or or… to see if I can find the best sound for the riff. I can do this immediately and avoid hours of trial and error. Also, theory knowledge helps you understand common/predictable ideas. You can use this to your advantage by finding quick progressions that work (for writing a lyrical song or lead), or avoiding common ideas and breaking the standard patterns.
My suggestion to you is this – learn basic music theory and how to use it in your improvisation and composition. Learn how to analyze your favorite songs and solo over them. Learn how to read basic music notation, learn a scale system such as the CAGED system, and learn how to move chords around the guitar in various keys. This will give you a practice set of skills for applying theory to the fretboard, open up many creative avenues for you, and enhance your ability to generate new music. Have fun with it!