For those who cringe at the thought of music theory, I would like to share my experience and give a tip.
First — you gradually learn music theory as you continue to play and write music. You don’t have to go to school and take a course, you don’t have to read a bunch of dry and boring books. It just happens as you continue to do what you love to do — make music.
For example, if you play guitar you will soon learn that songs are in keys. And each key has certain chords within it and other chords that aren’t included. And after that you learn that if you know some chords are major and some are minor, you can fit them in to spice up a song. Pretty soon, you will be interested in learning more types of chords to improve your music — like 7th chords, sus4 and even diminished chords.
Now the tip — to learn where all these chords fit in and how to know when to use them, I recommend learning the cycle/circle of 4th/5ths. The reason this has two names will be apparent when you learn them. The best tool to use the circle (so you can look them up and not have to memorize them) is a simple graphic you will find many places on the internet. If you don’t like the one I show you here, just search for the term (circle of fifths) and take your pick.