Write 500 to 1000 words as quickly as you can without any self-censoring. This is called freewriting. It will warm you up and begin to reveal themes that are foremost in your mind, which will become the ideas you can write about most interestingly. This need not be a major project -- simply type or write the thoughts as they come to you.
Find the themes, words, questions or ideas in your freewriting that most interest you. Don't worry about whether they make sense. Often things will resonate with you on an emotional or subconscious level. Even though you may not be able to explain them, they are almost always worth pursuing.
Sketch out a summary of your story based on the themes you've identified. If you are unsure how to develop them, keep freewriting with a focus on the ideas you have chosen. The story summary may be an overview of a conflict and a resolution, or simply a starting question or situation that a character must face.
Ask yourself questions about the main character and write down the answers. Focus on the values that shape the character's choices and on the qualities and experiences that have shaped those values. If certain aspects of the character don't ring true or reduce your emotional investment in the character, try altering the character to remove them. Choose solutions that seem believable and remain consistent with the character's personality.
Write a draft of the story quickly and without worrying too much about coherence, correctness or completeness -- you can fix and polish the story later. At this stage the main goals are to explore potentially worthwhile ideas, to create a character you love desperately, to develop a pressing conflict that will drive the character to a meaningful choice or effort, and to relay a convincing source of strength the character will draw upon to make that choice or effort. These elements reveal the character growth that lies at the heart of any interesting story.