You can't write a single decent line until the story comes alive in your mind. Take a few moments to imagine the details of each character- what they need, what they desire. What would they kill for, die for? When they start surprising you and getting into all sorts of trouble, you'll know you're ready to work.
Expand your vision to include the tiniest details of the setting. How does it look, feel, smell? Once you can imagine it, you can begin to create that world on paper for your readers. Start by peeping through a keyhole, and focus on one thing at a time. Gradually widen your perspective until it encompasses the room, the house, the whole world you are creating. Let the story unfold like a mind-movie, a private screening just for you, and write it all down.