Why learn Music Theory?
If you have ever questioned the importance of learning music theory on your way to becoming a real guitarist, or musician for that matter, check this link out first. Mike Phillipov explains why it is one of the best steps you can take.
 http://www.cyberfret.com/theory/mike-philippov/should-you-learn-music-theory/index.php

Musical notes
There are 12 notes that exist in all of music. There are Natural notes (A B C D E F G) and their chromatically altered semitones. The weird thing about the other 5 notes is that they can be called one of two different notes consisting of a “sharp” (#) or a “flat” (b), depending on a reference note. For instance, you may deal with notes named A#, C#, D#, F# and G#, as opposed to their other note names Bb, Db, Eb, Gb, and Ab. A# is the same note as Bb because they are both the semitone between A and B; they are called “enharmonic” notes (same note with a different name).

Tones and Semitones
You may be wondering why there are no Fb, Cb, E#, or B# notes listed above. This is because in music there are things called tones and semitones, or sometime called whole and half tones respectively. On your guitar, each fret is a semitone from the one before or after it. Every 2 frets is one tone away from the one preceding or following it. It so happens that if you were to count all of the natural notes on the low E string, starting with open E (fret 0), this is how you would find each note:
Semitone, tone, tone, tone, semitone, tone, tone
…and those notes respectively are:
F, G, A, B, C, D, E.
On your guitar neck you will notice that there are no notes between E and F or B and C. This is why there can be no E#, Fb, B#, or Cb. There’s no room! This is consistent over the entire guitar neck.

The Major Scale
Now that you know your notes and where they lie on the guitar (hopefully!), it’s time to learn the fundamentals of all music theory – the Major Scale. A scale is a specific pattern of notes consisting of arranged tones and semitones in ascending or descending order, creating melody. There are hundreds of scales in existence, but for the sake of simplicity we will start with one of the most basic and fundamental. We will also be using the “key” of the note C, meaning the note C is the first note and last note in the scale we choose, also called the “root note”. We chose the note C because the major scale on the key of C consists of only natural notes, again for simplicity.

Also tabbed…
E-----------------------|
B-----------------------|
G-----------------------|
D--------------7-9-10-|
A-------7-8-10--------|
E--8-10----------------|

What’s with the W’s and ½’s?? Like I stated earlier, scales are a specific arrangement of tones and semitones, or whole and half tones. The distance indicated by the W’s are a whole tone, or two frets distance from one another. The distance indicated by the ½’s are a half tone or one frets distance from one another. This pattern is specific for the Major Scale only! Within the first octave (eight notes), the pattern will always be W, W, ½, W, W, W, ½. The easiest way to remember the tone pattern for a major scale is two sets of W-W-1/2 separated by a W. In the major scale, this simple pattern can be applied to any eight consecutive notes. To prove this, we can try the 8 notes from G to G in major:

Notice the F# placed in the scale. This is there because in order to fit the pattern of whole and half tones we established, it must be placed there because F# is a whole tone above E. It works every time!
Now, you may have seen major scales that extend past eight notes at a time, ascending or descending. They do exist, but they are not as easy to learn as these simple octaves. There are also scales out there, including major, that begin, end, and are played on various other frets and strings on the guitar. This only makes sense because the same note can be found in many other places on the guitar neck, but the fretting arrangement will not be the same as the one above. Those are called positions of a scale, of which there can be many per scale. You will just have to explore a bit deeper into the millions of combinations of scales, keys and positions out there. You can also look at the scales portion of this website to learn more basic ones. Learning and practicing scales every day on your guitar is one of the most grueling, but also fastest and most efficient ways to lighting up the guitar neck like a freakin’ pro! So do that every day and see what happens, and have fun!