If you want to be caught in the trap of musical form/structure, comfortable progressions, conventional melodies & harmonies then start copying/changing what has come before.
Alternately you could start but opening BIAB by picking a style you like the sound of, changing some components of it to suit your taste and then bang in chords until you're like the sound of them together.
Far, far too many people are trapped by "the rules" of Western Musical tradition which is really only based on a couple of centuries of music that was controlled by the very rich and very powerful or you could, I suppose heed those who say learn the rules to break them, but it's a trap. Ignore the rules and mix things you like to make the stew you want.
UNLESS you want to work in an established style, with all of it's attendant requirements.

Cheers
rayc
"What's so funny about peace, love & understanding?" - N.Lowe