I'm no musician, but I do play the guitar once in awhile. Yet, many of my peers have come to me and ask me the same question every time: "How to play the guitar?" Well, I've never had any "proper" music education and was mostly "self-taught". So, this is the story of how I started learning to play the guitar.
I started seriously learning music after high school, which is quite late in my opinion. At first, I would reject playing any songs with a barre chords and would only play simple C-F-G-Am or G-C-D-Em songs, which are all open chords. After stumbling across a lesson on the theory behind the four chords, everything I learned started to make sense.
The first thing we need to know before knowing what the four chords is what is a chord?
What is a chord?
A chord is actually a harmony of a few notes. The most basic form of a chord consist of the root note, the third note and the fifth note. So for a C major chord, the notes of the chord would be the C, E and G. Now, most of us know that there are major chords and minor chords; and we would normally relate to them as a bright chord and dull chord respectively. Minor chords basically just drop the third note by a half-step. So for a A minor chord, the notes are basically A, C, E (A major: A, C#, E).
Why need the minor?
When a song is played in the key of C, or on a C major scale, you would be most likely be playing all the white keys on the piano (C, D, E, F, G, A, B). The basic chords of the C major scale would be C, Dm, Em, F, G, Am. These are also known as family chords as they are all related and consist of all the notes to the notes of the C major scale (no black keys of the piano from Do-Re-Mi-Fa-So-La-Ti). This is also why I would be using an A minor instead of a A major while playing a C-key song because C# is not in the scale of C major. This applies to every key in the major scale, so some would recommend calling chords by numbers (C=1, D=2 etc.), making it easier for transposing the song to a higher or lower key.
As I said, four chords is all we need. So, we would be using the C, F, G, and Am, also known as the 1st, 4th, 5th and (minor) 6th chord. The reason that these chords are used in most pop songs is because these four chords alone could cover every note on the C major scale. So, all we need to do now is find the chord that is in harmony to the melody. This now comes down to the timing of the music. Music is basically counted in beats, bars, and phrases. A bar consists of four beats, and each bar would usually have it's chord. Phrases (in modern day music) usually consist of four (sometimes eight) bars, indicating the completion of a musical loop. So, when we find the sequence of the chords for the first four or eight bars, we could already know the chord progression for most of the song.
Take my favorite song, Say Something by A Great Big World. The song is in the key of D, so all the notes and chords would be a whole-step higher, making it D, G, A, and Bm. The first four chords in the intro are Bm-G-D-A (or vi-IV-I-V), and so does the chorus. This is one of the most used chord progression in today's music (and also why do many songs just sound so similar). Even the great John Mayer wrote an article of it in Esquire here.
Learning to play music might seem to be rocket science for some people, but the songs that speak to your soul might only consist of no more than just four chords.