How does a poem begin?
A poem begins with a thought with interesting words to it. The nature of their interest can be that the words feel so appropriate: that their sound encapsulates what they describe. It can be that they’re interesting because they sound beautiful, but that’s not necessary. Maybe they’re interesting because they are appropriately ugly, or appropriately distant. Maybe they’re interesting just because they’re not words you’d hear every day, but still: they came up in that thought, fitting what they describe so aptly that they can’t be ignored.
If I follow the train of the thought, images bloom, and more words assign themselves. Sometimes I have a sense of where a poem will take me, sometimes I don’t. That’s what’s fun about poetry, versus narrative, where I’m usually steering. The form that the poem takes reveals itself. Even if I realize: this poem is a sonnet that will need to be completely restructured. I don’t usually begin by having that information.