There are two basic ways to develop the Story part of your Big Shiny Idea. The most basic is to simply sit down and start writing. Typically this is called "pantsing" i.e. writing by the seat of your pants.
Here are the advantages and short comings of this method.
Writing as the inspiration hits is a wonderful thing. But it can also be problematic. The big advantage is that it lends itself to more organic plot-development. A number of authors just like to write and see what happens next. It also can provide quicker turnaround between projects because a lot of the "preproduction" is put aside in favor of writing to see what the story is about.
Personally, I've written two novels and most of a novella this way. It's... all right. I also tend to spend a lot of time going back and inserting scenes and deleting massive chunks when it becomes obvious that the plot has taken a wrong turn.
But that's me. Writing in the moment may be the way to go for you. If it is, that's great. If it's not, never fear, I'll be addressing Outlines next time.
One word of caution regarding the pantser method. A lot of beginning authors seem to think that in order to be a "real" writer they need to write without an outline. I can't tell you how many years I spent thinking that I shouldn't need an outline to write a novel. (Well, I could but it's embarrassing so I won't.) During that time I started novel after novel and wallowed around trying to find the core of the plot, the characters driving motivations, before finally chucking the whole thing and starting something different. There is nothing wrong with using an outline.
There is nothing wrong with not using one either. You need to do what works for you. My caution is that I don't want you dismissing something without trying it out first.
As I've developed as a writer I've found that I have gotten better at pantsing. However, I still outline short stories and novels when I feel I don't know the plot/characters well enough to just write. Which method works best for me varies from project to project. It might for you too. And that's okay. The only real rule we need to adhere to is "Do what works for you."
Do you find as you write more that your pantsing ability gets better? Or do you crave the outline more?