There's no one way to start a poem
From an interview with John Gilbert in the Paris Review (The Art of Poetry, No. 91):
How do you start a poem?
There’s no one way. Sometimes I’m walking along the street and I find it there. Sometimes it’s something I’ve been thinking about. Sometimes it’s an apparition.
How do you know when you’ve finished one?
If I’m writing well it comes to an end with an almost-audible click. When I started out I wouldn’t write a poem until I knew the first line and the last line and what it was about and what would make it a success. I was a tyrant and I was good at it. But the most important day in my career as a writer was when Linda said, Did you ever think of listening to your poems? And my poetry changed. I didn’t give up making precreated poetry, but you have to write a poem the way you ride a horse—you have to know what to do with it. You have to be in charge of a horse or it will eat all day—you’ll never get back to the barn. But if you tell the horse how to be a horse, if you force it, the horse will probably break a leg. The horse and rider have to be together.
And some quotations regarding the writing of poetry:
"Poetry is everywhere; it just needs editing." - James Tate
"Writing a poem is a kind of hunt for language." - Jackie Kay
"Poetry is ... a kind of leaving of notes for another to find, and a willingness to have them fall into the wrong hands. " - Matthew Hollis
"Poetry is an utterance of the body ... It is the language in thrall to the corporeal, to the pump and procession of the blood ... " - Glyn Maxwell