Choosing the right chords for your song depends on 5 things. 1. Deciding how you want your song to sound. 2. Having an appreciation of how complex chords and harmony can get. 3. Knowing how chord progressions work. 4. Matching your songs needs with your understanding of harmony. 5. Trial and error - because music theory never tells the whole story.
It would be oversimplifying to say "just use C - Am - G - F," because all you've got then is a particular chord progression which works without understanding why and whether or not it's best for your song.
Far better to get a quick, rudimentary knowledge of why some chords work together and others don't. Then, I guarantee that your understanding of music will expand like small like small universe and a fountain of creativity will flow from within.
Heartbreak songs will usually feature a minor chord - or may even be in a minor key. This is because, in western music, the minor is usually interpreted as sad or mournful.
So we know how we want our song to sound sad but we don't want it to be sad all the time. So we will have to use a major chord as well. So we end up using what we call a chord progression or if you want a more accurate term, you could call it 'Harmonic Structure.' In other words, we move from one chord to another to enhance the emotions and gravity in what we're trying to convey.
You can write a pretty decent song with just two chords, ( some folks can even do it with one ) but mostly, a song will have at least two chords, so which one's ?
It depends.
It depends if you want to keep it simple. Or whether you want to add suspense or build to a crescendo, sound sinister or keep the mood flat and repetitive, make it higher or lower. You will feel this happening as you write your songs, you may do it without realising what you're doing.
This is because you already have a subconscious understanding of how music works - after all - you've been listening to it all your life.
And this is how you'll know if the chords you are using are right. Your inner emotional compass will tell you. Having music theory knowledge just gets you there quicker.
So just to prove the point, any musician will tell that a G chord will work with C Chord. Because there's an inherent relationship between them. So that musician does not have to go around hunting for a good sound.
Well, 12 semitones in an octave + minor variations = 23 options every time you progress to another chord. And that's without 7ths and 9th etc and things called modes which we go into here.
Yes that's ridiculous - which is why we usually to stick with chords in the key were playing in. This narrows it down quite a bit and we have a simple way of making sense of this for any key. What we do is this.
We assign Roman Numerals to the chords instead of letters. That way, the same chord structure or can be applied to all the keys and it will sound the same.
So instead of 'C Am G F' for our chord structure we write I - iv - V - IV
Here's a list of the chords you'd use in the key of C major.
C = I
Dm = ii
Em = iii
F= IV
G = V
Am = iv
Bb = VII
Well, it depends. If you stick to those chords in your key, as above, you will usually be able to make any combination sound good. As a rule of thumb, the chord progression should always follow what the melody is doing - not the other way round. In other words - don't write a melody to fit a chord progression, write a chord progression to fit the melody.
In practice, you'll usually find they reveal themselves at roughly the same time anyway, so don't worry about it to much.
Here's a list of standard 4 chord progressions which will almost always work, unless you have a really whacky melody. You don't have to use 4 chords - use as few or as many as you like, these are just very well used suggestions based on chords 1,5,6,4, to give you an idea.
Key of C Major
Key of C minor.
Key of D Major
Key of D minor.
Key of E Major
Key of E minor.
Key of F Major
Key of F minor.
Key of G Major
Key of G minor.
Key of A Major
Key of A minor.
Key of B Major
Key of B minor.
So which chords do you choose ?
The order does not matter that much and you can leave any of them out. Just let it take you where it needs to go and you will have the basis of a good song.
You may find the melody is all wrong or the song has completely changed in character. Well that's where the trial and error comes in. There are hundreds of variations to these chords if you look online but they all fit this pattern of relationships.
That's because it's all about relationships. I and V always work well. Next the relationship between ii - iv - vi always works well but not as great as I and V. Lastly, III and VII work less well and you will probably have to mess around with them a bit to get a good progression.
That said, you can use anything you like, even chords from another key if you can make it sound right. These are just the ones that always work.
Here's a list of chord progressions which are often used, but there are no hard and fast rules. If it moves people, it's working.
So try not to get too hung up the chords you choose - nor the chord progression - because they are not the song, they're just part of it. But they can make the song great and outstanding when combined with the right words, melody, singer, accompaniment, etc.