In the first long work of fiction I wrote, I made the classic beginner's mistake of starting the story too soon., with two chapters of backstory.
I wrote the first version of the book, "The Angry Little Boy," which will be published by 4RV sometime next year, in a weekend. Then I spent the next couple of years learning enough about fiction writing to make it publishable, including signing up for an online course on revising and editing. The first assignment was to post a chapter and revise it.
Taking a look at the first two chapters, I decided they would be poor material for the assignment, so I chose chapter three. It was one of those moments of clarity or perhaps sheer blind luck. Ultimately, with the help of the instructor, I cut out the first two chapters entirely. The necessary information, quite a bit less than I originally had, ended up as a flashback.
Determined not to stumble into the same pit twice, I searched for a method to determine where to start a story. Simply put, where to start is where the story begins, and where it begins depends on what the story is about, which means writing down the key concept.
In my story, a little boy loses his mother in a fire, but the adults around him are too immersed in their own grief to pay attention to him and help him with his. Formulated this way, it was clear that the action started when my main character arrives at his grandmother's house. His mother is dead and father in the hospital. The first two chapters, for which I had done quite a lot of research, were about the fire and her death, not about what happens after, and so I cut them.
While stating the core concept of a story may not be quick or easy -- it took me a couple of weeks of staring at my first few chapters to figure out the core concept for my current work-in-progress -- it serves as a guide to both where to start, and how to focus the story. Begin at the beginning, not before.