Most writers have a million ideas floating around in their head at any given time. It can become difficult to separate them enough to focus on one writing idea. If this sounds like something you experience, you should sit and narrow things down. Otherwise, you will waste a lot of time trying to work out how to go about discovering a great story idea.
Pick a Character
Start with picking your character and develop that person in your mind. Not so much their physical attributes, but their emotional ones. Do they have any emotional scars? What are they? What are their fears and concerns? Address these things in your story idea.
Plot their Story
From there, you can decide what they are dealing with. Were they orphans, are they in a relationship that they fear losing, going somewhere and ending up delayed? Once you have decided what they are dealing with and who they are, you will be better at writing it as a short story.
Ask Others for an Opinion
Online there are many writing groups that can help you figure out whether your story is a good idea or a waste of effort. Share with them, not the story as you have finished it, but the idea. Perhaps it is a woman who has always been afraid to commit to a relationship ends up jumping on a plane to meet what she thinks is the man of her dreams, only to end up with a delay or a crash or something major that slows her down. They will tell you if it is a good start or something that is better left as an idea.