What is music theory?
Most of my favourite musicians and guitar players CAN NOT read music (including Sir Paul McCartney, Angus Young, Dave Grohl, Eric Clapton and even my personal favourite film composer of all time - Danny Elfman).
So do they know any music theory? You bet they DO!
To be able to write a song, jam with other musicians or play a guitar solo - you MUST know at least some music theory.
Having an ability to read traditional music notation IS NOT essential to do these things
So what do you need to know in order to jam with others, improvise a solo or come up with a piece of music that works?
I will be doing SO MUCH MORE on this topic in the future. I will even provide a music theory course, giving you everything you NEED to know, and nothing that you don't!
For now, don't think that you HAVE to learn to read music to make progress (or to start!).
There are many important things that will help a beginner to quickly learn to play guitar. In my opinion, learning to read music notation is NOT one of them. In fact, learning to read traditional music notation will only make learning to play guitar much harder and take far longer for a beginning guitar student!
Beginner guitar students need to focus their attention and learning on the physical demands of learning to play guitar and developing the relationship between these physical skills and the sound they create (ear training).
If they also focus on the demanding task of learning and memorising music notation, then the student's entire progress slows down - which leads to massive amounts of frustration with little benefit in the early years of playing.
As I say, most of my own personal guitar heroes including Angus Young, Jimmy Hendrix, Dave Grohl and Eric Clapton never learned to read music. They definitely attained a tremendous amount of skill, musicianship and practical music theory knowledge (scales and keys, etc). But, learning to read music notation is something they did not require for what they went on to do.
Of course, learning to read music notation can be a valuable skill, however it should be learned later in a student's development if required, NOT as a beginner.