Since the thread's here... this is how to make harmonized chord progressions.
This is coming from a guitar point of view...
Chordsare 3 or more notes played at once.
Generally chords are 3 or 4 notes, but can be expanded to anything you want.
Take the notes in the C
major scale:
C D E F G A B
Those are the white keys. The major scale always follows the same pattern, whole steps (two keys) betwen each note except between 3-4 and 7-1, ie in the key of C, E-F and B-C. C is the 1 OR the 8 since the scale repeats.
There are 7 'n' (n=3, 4, 5, 6...) note chords that can be harmonized in every major scale (aka
diatonic). They ALWAYS follow the pattern I=MAJOR, II=MINOR, III=MINOR, IV=MAJOR, V=MAJOR, VI=minor, VII=diminished, which is a weird sounding chord that's typically ignored.
So, in C major, take every other note:
C E F = C major
D F A = D minor
E G B = E major
F A C = F major
G B D = G major
A C E = A minor
B D F = B diminished
(I can explain chord spellings if someone wants it, or you can take my word on it, no big deal, I'm glad to teach what I've been taught)
Playing those notes at once creates chords that go together in the key of C major. Adding the 4th note makes them all major 7 or dominant 7 chords. Major chords are more 'solid', minor chords are more emotional, and 7th chords add tension. Generall speaking you want to start/end on the chord in the key of the tune, ie if it's in Bb, start/end on Bb.
BTW C major is the same as A minor (exact same notes, different point of view, tonal center, tonality), so you can compose tunes around a minor scale just as easily with the same tools.
Anyway, the beauty is that you can pick between these chords and they're 'guaranteed' to work together. There are 12 keys to work from too. C, E, G, Bb for horns, F are the most common. It's also good to just play the black keys, who care what key it is

.
Anyway, it's a deep subject, but learning it piece by piece isn't so bad and goes a long way in composing music.