It really varies, sorry, I know that's not helpful. Usually at the moment it's the words, though sometimes it can be a chord progression that just brings words to mind. Often the chorus will come first, then I write some verses just to accompany it really :-) Some parts will go on the back burner, like I have loads of basslines that may one day be used, several choruses, and loads and loads of poems that may one day be songs. It's like having a box full of pieces from several different puzzles. I don't think there are any rules really, apart from that it should sound good to you as the creator ?