Every guitarist has holes in their playing skills.  If you've ever felt like you're starting over everytime you learn a new song or you can't improvise to save your life, you've probably just missed out on some basic music theory along the way.  Learning and playing the guitar can be made considerably easier if you know how the music is put together.  Let me explain...

First thing... You don't have to learn super advanced, esoteric music theory to get the job done.  Knowing which chords you'd play a Phrygian-Dominant scale over isn't important to learn unless you want to and the music you're playing needs it.  What we're talking about here is things like how you create a major chord.  Trying to learn an instrument without getting a little theory under your belt is like trying to learn a language without learning the grammar part.  You'll be able to butcher your way through enough to find a bathroom but you won't be having any deep conversations.

I know when you think of the word "theory" you picture a mad genius in a lab inventing things that us mere mortals can never hope to understand and then putting in a book that's all drudgery and no fun.  Music theory IS NOT that.

Think of it this way... Music theory is simply the common patterns that we see in western music. 

Music has tons of patterns.  Going from one song to the next, you'll see the same musical elements used over and over, if you know what to look for.  If you can recognize those patterns you'll be able to take the knowledge you get from one song and apply it to the next one.  And you'll never feel like you're totally starting over.

You could, of course, just keep battling your way through song after song and eventually find those patterns by yourself.  But that takes a long time and a lot of work.  Wouldn't it be easier if someone just laid them all out for you in the beginning?  Lots less work.  Lots more enjoyment.

I made the analogy to language earlier, but in music the theory IS our language.  And being able to communicate to other musicians in anything more than "put your finger here" (the equivalent of grunting in music communication) will be very helpful to you any time you're playing with others.  So the next time your friend wants to jam a 12 bar in the key of F# with a turnaround and a quick change, you'll know what the heck he's talking about.  Oh, and all those terms I just mentioned would take you about 10 minutes to learn.

From the "I just want to play, man" crowd, I will often hear Jimi Hendrix used as an example of someone who didn't have formal training and couldn't even read music.  Totally correct.  That doesn't mean he didn't know the theory behind what he was doing though.  He learned it in the trenches of the chitlin circuit, playing hundreds of gigs, and learning from the other musicians around him.  And at all times his reputation and pay check where on the line.  It's not the easiest way to learn unless you're willing to road dog it for a bunch of years and do absolutely nothing else but play your guitar.

While Jimi may not have had formal training, he still knew what he was talking about and was able to communicate with other musicians.  He knew that a C chord and F chord work well together and could probably reference a dozen other songs using the same changes. But he put in a lot more work to get to that point than you need to.

Will learning music theory destroy your creativity?  No, of course not.  It's music Theory, not music Rules.  These patterns get used over and over because they work, not because some music policeman will arrest you for doing something different.  You really can do whatever you want musically.  The funny thing is that even when you think you've invented something entirely new, chances are there's already a theoretical explanation for it.

That's the real point.  Nobody is so over the top creative that they can completely reinvent this art form of music that's been developing for hundreds of years.  Music just morphs from one idea to another.  New ideas created from old ideas.  Every guitarist and songwriter has their influences.  Everyone is using the same patterns.  But absolutely do your best to break the "rules".  That's how we find those new ideas.  It keeps things fun. But you have to know the rules before you can break them.

Bottom line, music theory isn't scary or hard.  It's just learning the patterns of music at the beginning so you don't have to figure them out on your own as you play.  You'll learn songs faster, jam better, communicate better, and have more fun playing your guitar.

Where do you start? Grab a book like Music Theory For Dummies or a theory workbook for little kids.  Keep it simple at first.  And don't be boring and just work straight through the book.  As you learn a new trick or concept go look for it in songs you already know how to play or are starting on.  It's much more enjoyable to learn this stuff in context so you can see that you're already using it.  And eventually you may WANT to learn which chords to play a Phrygian-Dominant scale with.  Get to it!