1. WHO? Who is the story happening to? What is your main character's name? If you are writing in first person, have you found a way to let the reader know who "I" really is? What do you know about this person? What does she like? What is he good at? Who are is friends? What does she want more than anything?
2. WHAT? What is going happen to this person? Will it change her life? Will it put his life at risk? What will he or she do next? The nuts and bolts of any story start to develop when you ask yourself what else could go wrong? What would your character do about it?
3. WHEN? Does your story take place in the past, present, or future? What specific year? Who is the president? What are the styles? Be sure to consider small questions, too, like what day of the week it is. What time of day? It will matter for what your characters are doing, what the light is like, if they're out past curfew, or up earlier than usual. Time details matter a great deal when you are describing the world your characters inhabit.
4. WHERE? Does your story take place in a small town? A large city? Outerspace? On an oceanliner? In each scene, are the characters indoors or outdoors? In a place that's familiar or unusual? Try to see the world through your characters' eyes--what do they see and smell and touch?
5. WHY. This is especially important. What is motivating your characters? What makes your hero want to save the world, or rescue the princess, or run into the burning house to retrieve an object? What are his or her defining traits, that make him or her act they way s/he does? What is holding her back? What makes him rush forward, even when he's afraid?
Oh, and about that H. HOW do you use these W's in your writing, you ask? Well, that part's up to you!