Now you have your idea and it seems that there could be a story. Then you begin to wonder. Is It Going Somewhere? Just what does this mean. There can be stories without plots but having a plot makes the story easier to write. Now comes the time for your imagination. You may already have the very basic plot. Could be girl meets boy, girl wins boy or man is harmed man finds redemption. These are very simple plots.
What I do is when an idea sparks is perhaps strange but for me it works. The new idea for a story has come. At night as I'm falling asleep, I start telling myself a story. This usually begins with the opening scene. Each night, I take this scene and while falling asleep add other scenes. Now this doesn't always work. Some ideas have a great opening scene in my thoughts but little else follows. Sometimes this process works and I sit for several days writing short sketches of what scenes are possible. Not all of these scenes might make the final story but they have given me spin-off ideas that I can use. So this is the third step in testing the idea. You may not use the falling to sleep scenario but when you have an idea, think about what scenes you will need to progress the story and also consider the settings and characters.
Knowing where you are going, even if the journey takes side trips, the stories will flow better.