Write a brief synopsis of your story, no more than a page or two, covering the key events of the plot. You can break it up by chapters if you're writing a novel, or simply by incidents in the story. The particulars may change as you compose the draft itself---inspiration can lead you in surprising directions---but the synopsis keeps the basics clear in the early stages of the process.
Compose a one- or two-page description of the main characters in your story. Include each character's personality, history, relationships to the other characters and ambitions or goals. A physical description is good, too, but try to focus more on the characters' internal lives than their surface details. (Their personalities will play a larger role in the plot than how they look.)
Find a good place to write. It should be quiet, comfortable and free of distractions. Ideally, you'll have a place in your house that fits the bill---equipped with a soft chair and a reliable writing surface---and that doesn't have any TVs or video games around. If you don't have that, go to a public library or similar quiet location. You can write there with a minimum of distractions as easily as you would at home.
Prepare a schedule for writing: a specific time each week when you'll sit down and compose your draft. It can be every day if you wish, but it should at least be once a week. Make sure you have no other commitments which may crop up during that time: writing is a habit and it only works if you keep at it.
Write your draft in the space you have set aside for it. Don't concern yourself with going back and fixing things, or even checking for spelling. Just get the story down on the page as completely and thoroughly as you can. Refer to the notes you compiled in Steps 1 and 2 to keep everything clear, and don't worry about meeting a word count goal. Some writing days will be more productive than others, and you'll discover your best pace as you go along.