I'm not about to explain WHY you should learn theory or tell you you NEED to, I'm just gonna put the information here and you can all make the decisions for yourselves and follow your own philosophy. So here it is, true, pure theory:
Section 1--Pitch, Rhythm, and Timbre:
These are the 3 qualities of music.
Pitch- How "high" or "low" the sound is perceived (play a note, now play a different note, and the reason they sound different is there's a different pitch).*
Rhythm- When the sound happens in relation to time (sound "A" happens 2 seconds before sound "B" for example).
Timbre- The type of sound (the reason a guitar sounds different from a snare drum or a piano).
*Scientifically speaking, the reason sounds have different "pitches" is because the soundwaves are travelling faster or slower. Slow, spaced apart waves sound low. Fast, close together waves sound high. http://www.physicsclassroom.com/class/sound/u11l2a.cfm
Section 2--Cents and the Octave
Differences in pitch are measured in "cents." So you could say "This pitch is 20 cents higher than that pitch." One cent is so small, that if you heard a sound, and heard another sound 1 cent higher or lower, you'd think they were the exact same sound.
Thousands of years ago, when people were first studying sound and how to make sounds that sound good, just about every culture in the world working on this simultaneously discovered (remember, back then they couldn't e-mail each other and let them know. Everyone pretty much thought that their civilization was the only one in existence) that if you make a sound, and make another sound about 1200 cents higher or lower, they will have the same effect in the context of the music. They called this interval (an interval is a distance between 2 pitches) an "octave." Play an E, then a B, then an A on your guitar (or whatever instrument you play). Now play the same melody* an octave higher. You'll notice that other than the pitch difference, the melody sounds basically the same.
*Melody- a series of pitches used in a musical piece
Section 3--Twelve Tone Equal Temperament
Cultures that had discovered the octave began to divide the pitch inside the octave into a certain amount of "notes". So One culture may have divided the octave into 10 notes, each 120 cents apart (120x10=1200). Western cultures developed the "12 tone equal temperament system" or "12TET" which means there are 12 notes each equally spaced apart in pitch. This system is the prevailing system of notes today, most likely every piece of music you have ever heard has used it. The notes each have a name:
A# or Bb
C# or Db
D# or Eb
F# or Gb
G# or Ab
A (the octave)
# means "sharp," and b means "flat." So "D flat" is written as "Db." When you move a note down 1 semitone*, you "flatten" it, and when you move it up 1 semitone, you "sharpen" it. Notice there is no sharp or flat note between C and B, and no sharp or flat note between E and F. In that way, sometimes E will be called "Fb" or F will be called "E#" or B will be called "Cb" or C will be called "B#." It depends on the context, which you'll learn later.
*semitone-The distance of 100 cents. Each note is a semitone away from its neighbors. C is a semitone away from B, B is a semitone from C, A is a semitone from Gb, D is a semitone from C#, etc.
Section 4--Sheet Music and Rhythm
I can't figure out how to post pictures here, so I'm going to leave the job of teaching you "notation" (how to write music on paper) to these guys:
For the purpose of basic notation, you really only need to watch the first few videos, but I suggest you watch every video on that page and pay CLOSE attention. It's good to learn music theory from a variety of different sources (as long as their trustworthy sources) instead of just me. I'm going to continue the lesson assuming you've watched all the videos under the categories "The Basics" and "Rhythm and Meter." DON'T SKIP ANY VIDEOS, I'm not teaching basic rhythm in these lessons or anything about notation and those are both VERY important concepts to grasp and those videos explain them PERFECTLY.
Next lesson will include melody (tips for writing good melodies as well) and the general theory of harmony. This one was the most dry because its the very basics and I like to cover some acoustics (meaning the physical science of sound and how sound works) so next lesson will be more interesting and applicable to music.