Find an idea. There is no magic answer to finding a story idea. Ideas can come from anywhere. watch the news, read other writers and play the "what if" game. What if a small town was terrorized by a giant shark? You'd get Jaws. You can apply this method of searching for story ideas all day. It's fun and can be very fruitful. Keep a notebook of title ideas and story ideas for those times when you feel dry. Once you have an idea you feel will make a good story, it's time to begin the process of writing.
Start with a good outline. An outline may seem like an unnecessary extra step in the process, but it can be time saving in the long run. The outline doesn't need to be extensive. An outline for a short story can be as simple as a few numbered steps that give a synopsis of the major plot points in the story you want to write. The outline you write for a book will be longer, but again, you can simply outline the major plot points in your story that need to happen for your story to come together cohesively. Keep your outline handy for reference while you write your story, but be flexible with it. Sometimes a story changes as you write. Characters may seem as if they have minds of their own. Be open to a change in the story's direction. Your outline doesn't need to be set in stone.
Focus on the story during your first draft. Many beginning writers get caught up trying to perfect each sentence, or they keep going back over what's been written. This takes away from the forward thrust of the story and it can dull your initial enthusiasm for the story you want to tell. Write in the heat of the moment, while the story is still fresh and exciting to you. Making changes and corrections to the story is what a second draft is for, once you have the basic story safely down in front of you.
Read your story for structure problems, scenes that don't make sense, character development, and dialogue that seems flat or contrived. Make changes in your second draft, then read the story again for simple typos, grammatical mistakes and spelling. Don't rely on your word processor's spell-check function entirely. Words can be spelled right, but not for the context of your story. A word processor won't recognize that you should have used "two" instead of "to." When you're satisfied you have your story, read it again, this time for the pleasure of reading it.