Warm up by writing 500 to 1,000 words as quickly as you can, without worrying about what comes out. The key is to keep your fingers moving. If you're stuck, write about how stuck you are. If you hate typing this fast, type about that. If you think you're a brainless hack, tell it to the page. But don't stop typing until you're done. If you prefer a time limit, set a timer for 10 to 20 minutes and don't stop typing until it rings.
Choose the theme, word, idea, character or question that most intrigues you from what you just wrote. If nothing catches your interest, keep writing 250 words at a time until you hit something that does.
Turn the intriguing idea into a sentence in the form "(Character) wants (Goal) because (Motivation), but (Conflict)." At this stage it doesn't matter whether the story holds up, but it must have all four elements filled in. For example, if you detected intriguing themes of mice and mysterious books in your free-writing, your sentence could read: "A sneaky mouse wants to learn magic because it will let him become king of the mice, but the magic books are hidden away."
Ask "What if ...?" questions to improve your one-sentence story idea. Your mission is to create a main character you love, a very important goal with an admirable motivation, and a conflict that will force significant character growth. If you are having trouble getting started, try substituting key words in the sentence with their opposites, and keep the change every time it is an improvement.
Continue cycling through free-writing, questions (especially what if, who, what, where, when, why and how) and thoughtful editing to develop a setting, additional characters and, if the scope of your story allows it, subplots.