Many movie script writers (my brother John for instance) and many novelists begin with a "logline". That is the story distilled into one or two sentences that says who confronts what conflict. The logline will show you, and any producer or publisher to whom you pitch it, if your idea can be developed into a story of compelling interest. As you write the novel or script, keeping the logline in mind will help you to focus on what is the story and to leave out what is not.
Here, from the imdb website, is the logline for the remake of THE BEGUILED, which originally starred Clint Eastwood: "At a girls' school in Virginia during the Civil War, where the young women have been sheltered from the outside world, a wounded Union soldier is taken in. Soon, the house is taken over with sexual tension, rivalries, and an unexpected turn of events."
And here is the TO KILL A MOCKINGBIRD logline: "Atticus Finch, a lawyer in the Depression-era South, defends a black man against an undeserved rape charge, and his children against prejudice." The movie was adapted from a novel.
Once you have that seedling of a novel, you can guide its growth and development using such tools as summarizing, outlining, mind-mapping, timelining, and so on.
If your heart, intuition, and imagination pull you in a different direction, change the logline and the outline.
My favorite outlining software is Workflowy.