The chord progression within a key is a succession of chords that concur with each scale degree. In a word, each scale interval or degree has its corresponding chord. We are going to study the chord progressions in a major key and in a minor key. Let’s start from a major scale, for example the C scale:
C D E F G A B
We assign a chord to each interval, just as follows:
I -> major chord -> C
II -> minor chord -> Dm
III -> minor chord -> Em
IV -> major chord -> F
V -> major chord -> G
VI -> minor chord -> Am
VII -> half-diminished chord-> Bm7b5 (forget about it)
All right, well, if we are in the key of C, any of these chords could work just fine (some better than others), but we would always be playing within the harmony. If for example, we decide to play a F#m or an E, we will immediately notice that it doesn’t fit in, our ear will warn us that something’s wrong.
If we take the sixth degree of a major scale, we know that we get the relative minor, in this case Am. So, if we change the order, we will end up with the chords from the Am key.
For your early aspirations, you simply need to know that some chords are more important than others, besides the tonic chord, obviously. These chords are the IV (called sub-dominant) and the V (called dominant). You will see and confirm that a lot of songs are based on these three chords, tonic, sub-dominant and dominant. In a major scale, these 3 chords are major. In a minor scale, these 3 chords are minor.
We can make our chord progression more elaborate using seventh chords instead of triads. In such case, the progression would turn out this way (we are still in C major scale)
I -> Cmaj7
II -> Dm7
III -> Em7
IV -> Fmaj7
V -> G7
VI -> Am7
VII -> Bm7b5
This relationship between the 12 tones found in western music is represented thanks to the famous circle of fifths.