The puzzle of documentary photography is to crack the code of storytelling by critically investigating and finding expression for your narrative. Critically looking at a particular idea or subject matter with a solid insight based on experiences, then an already dissected matter with no standpoint. Finding your own story to tell without telling some already narrated stories but rather through exploration and experiencing life with people who own these stories will help you discover many other interesting yet untold stories that are unique from others.
Every photographer distinctively tells different stories even when they have some experience at the same time and space but the way each one will interpret their story either by a lens or by writing actually will differ.
As a photographer, you should start off with something that pricks your interest as a story to tell but on a base of curiosity which will lead to you asking more questions.