There are three ingredients you should have when writing a story: An idea, a plot, and a story question. You can come up with these in any order, but from a structural perspective, they usually go like this: idea leads to story question leads to plot.
When I start working on a story, sometimes I have a plot, sometimes I have an idea, and sometimes I have a story question. And sometimes I have a combination thereof.
An idea is a “what-if?” I generally look at it as defining the setting of the story. A story-question is what most people think of as an “idea”. Every idea can allow many story questions. A plot is a “then this happens”. Every story question allows for many plots.
You can start off with any of these, but you need to figure out all three somewhere within the writing process. Let’s look at an example.
Here’s a hypothetical idea: “What if a nation in a world equivalent to near-future Earth is in danger of being economically marginalized by a super-national economic/political unit similar to the UE, and it attempts to restructure its school system to produce valuable skills and professionals to help it compete?”
Now you need your story question. Let’s say: “How would this affect the students within such a system?”
A basic plot might be: “Smart students in a prestigious school do ‘bad’ things.”
This is from a story I am actually working on. I picked it because it demonstrates the loose order in which you need to come up with each element. I came up with the plot first. Then I came up with the story question. I wanted to know how current educational practices in various countries might affect the way children developed and behaved as students. How would the pressure to achieve affect various types of people, and what would they do to lessen that pressure?
I came up with the what-if last. Since I was looking at current systems of education, I needed a setting that could incorporate them. Since I was looking at the extremes of these systems, I needed slightly more overt pressure on the country to adhere to these trends.
But it’s completely possible to come up with the what-if idea first. A lot o sci-fi works on this model. A lot of epic fantasy or romance starts with a plot. A lot of serial UF and mystery begins with a story question.
It’s also important to keep in mind that you don’t have to begin with something in the plot continuum. What we’re looking at here is mostly the plot angle of attack. You could also start with a character, or a scene, or a setting, or whatever.