You want to write a story. Great! But the problem is that you're stuck before you've ever even managed to get the first word down on the page. You're just being taunted by the white page (or screen, as the case may be) in front of you.
If you haven't already, you may want to look into getting your thoughts organised. Figure out what you're going to write about, before trying to write anything. This may mean anything from making a few notes on a page to writing down every single thing that pops into your head, whether or not it's immediately relevant (my preferred method). With a more general (or even a very concrete) idea of where your story is going to go, the words should hopefully come a little more easily.
If that fails to work, try putting on some music. I tend to find that just putting on any old music will actually be counter-productive. Instead, try to find some music that matches the tone and mood of what you're wanting to write. Build playlists consisting of songs that remind you of your characters, or the theme that you're writing.
On a similar vein, open a window. Even if -- especially if -- it's raining. Open a window and put music on. And above all else, turn off the telly. Get away from video game noise. Avoid anything with a cadence (dialogue) or sound cues. But the window will do two things give you an ambiance, and give you fresh air.
Still nothing? Go somewhere else. Even if that means just rotating everything in your office 90° (I like to do this once every two or three months). Replace your office chair with a wingback or a barcalounger. Put your computer in front of your sofa. Get a hammock. Go lie down in bed. Migrate out to the back garden. Find a quiet coffee house or a diner. Most diners, especially between the hours of 10pm and 7am, don't mind if you sit around for hours on end. Just, if you do, be sure to leave a generous tip. Do it enough times, and you may even start to get free stuff (I used to practically live at a diner for this reason).
Still nothing? Clean your room (or office). Again, the effects are two-fold. You're getting your blood flowing, and you're getting rid of any annoying clutter or mess. This also sort of works as a surrogate for moving to another location. It's hard to concentrate when you're staring at J20 bottles and Mars Bars wrappers.
If you're still coming up empty, make a snack. Get some tea, or some coffee. Get something to eat. But nothing that requires two hands. I like sandwiches. Sandwiches are awesome for writing.
If the white page is still taunting you, try a writing exercise. Especially if it's nothing to do with what you're actually writing. Just a quick something to get your brain cells working. Find a prompt and spend five minutes just writing down what the page wants to have written on it. Ramble. Don't worry about making sense; just write. It probably won't ever see the light of day, but sometimes you get lucky, and manage to kick start the thing you actually wanted to write in the first place.
And if all else fails, start somewhere in the middle, and fill in the blanks. Work backwards. Just get words down on the page. Again, just let the page tell you what it wants to see. Personally, I loathe writing out of order, but even I've been known to go this route in times of extreme distress.