The only rule in songwriting is that you must understand what you are doing. After that, it's up to your audience, if you are good enough to attract one.

Don't go to classes unless you really have to. Listen to everything you can, very closely. Ask yourself what songwriters whose work you enjoy are doing. Work out how they are doing it. Experiment. Buy a large wastepaper bin, and don't be afraid to use it.

Hardly anybody can survive by writing songs for a living (especially not "folk songs" whatever that means) though you might make a little money from time to time, eventually, if you work very hard at it. Don't give up the day job.

Oddly enough, that isn't meant to be discouraging. Stick with it, and if you acquire the knack you'll never be bored for the rest of your life. And you may strike lucky.