Basic Questions

It's easy to say "I have a cool idea for a story" when what you mean is "I have a cool idea for a setting" or "a character" or "a situation".  Just the "imagine if you saw an elf hitchhiking" or "imagine if the elves are the same as vampires" or "what would happen if the Byzantines discovered the New World" is not a story idea, it's merely the germ for a story idea. You need:

• whose story are you going to tell? Pick the most interesting character, otherwise you'll get the "Johnny Depp isn't the leading man" syndrome where no one remembers the leading man. Or, if you need to do a lot of exposition, pick someone young, or new to the area, so we can learn as he does. 
• what does he desperately want? This will be something like "true love" or "father's respect" or some other strategic goal. Ideally he should have two goals, because then you should make them come into conflict. "I want to make a lot of money and have my family be happy" leads to "do you spend more time at the office or at home?" dilemmas.
• what does he have to do to get it? He often does not know this at the beginning of the story.
• What are the obstacles, costs and consequences of doing what he has to do?