This poem could grow out of a unique moment, like Walt Whitman's "When I Heard the Learned Astronomer." Or it could grow from a stage of your life, Like A. E. Houseman's "When I was One and Twenty." Or maybe you'd like to explore a repeated ritual, like "When I lift the trash can lid . . .."
The moment or incident you choose to explore matters less than the process of probing and unfolding. Delve into the "when" part, then work toward whatever resolution or closure feels appropriate.
Your poem should be between 8 and 16 lines, with at least two stanzas.