I have never started a poem yet whose end I knew. Writing a poem is discovering. -- Robert Frost
This is very true when writing poetry. As far as I know, no one structures a poem until after it is drafted.
This is true to my own writing of fiction. Sometimes I have an inkling of the end, but the true discover is in the writing process.
I begin with a thought, an idea germ. Through free-writing, the germ grows into a story. During writing, the story takes on life. The story doesn't take on true form until the editing process.
How do you discover your story?