Richard gets a "bravo". Being an old school guy, I believe KNOWING music is the framework for PLAYING music. Learn how notes relate to each other. Learn that no matter what note you start on, a major scale is whole step, whole step, half step, whole step, whole step, whole step, half step. More important, learn what a STEP is. Once you know how to look at a keyboard or guitar neck and know how to form a scale, you will never need a chord wheel again. The "Circle of 5ths" and "Counter clockwise circle of 4ths" will be ingrained.

Chord relationships come from practice and experimentation. You can sit and crank out status quo songs or you can experiment and play with chord progressions that are not "standard". If you are in C, standard composing sends you next to Dm, Am, F, G.... why not take it to Ab? Or F#?

Remember, songs are nothing more than stories set to music. You are telling the story. You can tell your story any way you want to. It all starts with fundamentals. An 8 week music theory class will open doors for you that will remain locked if you try to figure it out yourself. You will eventually figure it our yourself, but having a teacher there to answer questions will help a lot. I don't know how old you are, where you are and what your life is like so finding classes at a time you can attend them may be an issue, but for 8-ish weeks or so of sacrifice, you will get a foundation of knowledge that has value you can not put a price on. I was fortunate to have done all of this at age 5. I sat for 6 weeks before I was allowed to touch an instrument and took music lessons on a blackboard. And years later, had the man who taught me not been long since dead, I would have liked to find him and thank him. Seriously consider this. It will help you more than you know.