I know this might be considered a very vague question, but I don't know where else to go. I've started songwriting recently and I'm still a beginner. I was wondering, what key(s) should I write my song in, in order to express a certain viewpoint? For instance, I'm writing a song about wealth and industrialization and the cruelties that come along with it. In this case, what keys should I use to express my theme topic? What chords should I use in my chord progression? Should I use major, minor, diminished, or other chords and scales?
As alephzero says, there are no real rules about this, apart from being able to observe what other people have done - and if we follow their rules, we'll repeat the work they've done, which may be a bit pointless!
A few things to consider:
Do you have lyrics that express your theme? If your ideas about cruelty are expressed in the lyrics, you may have more flexibility in the harmonies you use. A couple of examples of songs about misery that use major keys are :
The Human League - I Am The Law
Bruce Springsteen - Born in the USA
Talking Heads - Don't Worry About The Government
Don't feel you need to stick to the notes of a single key. You may know this, but a key is only a very rough guideline to a song's tonality - it may be that you need to change keys in the middle of a song, or use chords or notes from outside a key, to give a sense of motion and emotional pull to the song.
Consider consonance and dissonance to express emotional movement, as well as the particular key and chords you're using. (The human league song, while simple, uses dissonance markedly).
If you're writing a song in a particular style, you may wish to consider using the chords that are part of the language of that style. A country song about the cruelties of life might use very different harmonic language to a drum and bass song about the same.
Consider using timbres to make your point, as well as just chords. Though it would be a bit cliched, you could use some very clangorous metallic sounds to represent the industry, and have a sweet human voice struggling to be heard through the noise, and not even use a prominent chord progression. A song that's a bit like this in the verses, using a fast Latin rhythm to show the chaos of New York (though it uses a clearer chord progression in the chorus) is Hell Of A Town - Joe Jackson:
There is no 'should' with this in songwriting. Using equal temperament tuning, which most instruments do now, each key is as good or bad as the other 23 - majors and minors. If it's a song to be sung, you need to be aware of the range, not too high or low notes.
As far as harmonies and chords go, in any key, there are three majors and three minors, leaving a dim to bring up the rear. Some songs will only have three chords (some only two!) but lots have four or five - and more. If your song modulates, there could be even more. Number of chords does not equate to greatness of song!
It's generally thought that minor keys do make something sound sad or serious, but that's subjective. You just have to try out as many ideas as you can, until something clicks as the best one. There are guidelines to this, but few rules. The 'sound good = is good' works well.