If you’re going to compose your own music, one of the first things you must do is to create the chords for your song. One of your problems will be finding the right combination of chords, and what chords to start or end your song.
I’m going to share with you a basic guide on how to lay out the chords for your song.
The Key
You must decide first how you want your song to sound. Is it a happy, lively and upbeat song? Or is it a sad, gloomy song?
If you want to create a happy song, the common choice is to use a major key. If you want a sad song, you might want to use a minor key.
But what exactly is a key?
In layman’s terms, the key of a song is the song’s "home pitch" and "overall mood". The "home pitch" is one of the following: A, A# (also known as Bb), B, C, C# (a.k.a. Db), D, D# (or Eb), E, F, F# (or Gb), G, and G# (or Ab). The "overall mood" is either major or minor. Examples of keys are Key of G Major and Key of A Minor.
If you want to know how a major key and a minor key sound like, try the sample audio below:
Scale in Key of D Major
Scale in Key of D Minor
Before deciding which chords to use, you must pick first a key (the pitch/mood combo), because the key determines the exact chord set you can use.
The Chromatic Circle
Before we discuss the chord set that goes with the key, it is helpful if we put all the pitch names on a circle. We will call this circle the chromatic circle (my terminology, hehehe…
).
The blue arrow represents the direction of ascending pitch. Each two consecutive pitches on the circle (e.g., F and F#) are one semitone away from each other. Similarly, the "distance" (interval) between G and A# is three semitones. Note that the sharped pitches have alternative names (e.g., F# is also Gb).
The Chord Set
Now for the actual chords.
Each key comes with a chord set. There are chords that define the key and therefore the overall sound of your song. To know which chords go with the key, we will use visual patterns for choosing the chords.
For the major keys, we will use the following pattern:

The green boxes are the appropriate chords you should pick for your song. The letters above the green boxes indicate that the corresponding chord is a major chord (M), a minor chord (m), or a diminished seventh chord (dim7, used rarely). The letters below the green boxes indicate the tonic chord (T), the subdominant chord (S), and the dominant chord (D).
Your task is to fill up the boxes with pitch names. Start first with the key pitch, then fill the rest of the boxes with consecutive pitches in ascending order (refer to our chromatic circle above). For an example, here are the boxes for the Key of D Major:
The chords that go with the key are the chords in the green boxes. In the case of Key of D Major, the chords are D, Em, F#m, G, A, Bm and C#dim7. The tonic chord is D, the subdominant chord is G and the dominant chord is A.
The process is similar for the minor keys, except that we will use the pattern below:

How To Begin and End A Song
There are many ways to begin and end a song. The popular choices are to begin/end it with a tonic chord (the most popular choice), a dominant chord or a subdominant chord. You can, say, start your song with a tonic chord and end with a dominant chord, if you want. These are not hard and fast rules anyway.
For a real-world example, the song Bed Of Roses by Bon Jovi is written in the Key of F Major. The song starts with a subdominant chord (Bb) and ends with a tonic chord (F). Hear it for yourself!
Bed of Roses by Bon Jovi
Stay tuned for Part Two!