If you're paralyzed by the thought of writing a first draft then try this simple exercise:
Grab a notepad.
Set a timer for ten minutes.
Jot down the scraps, bits of dialogue, images you have simmering in your head.
When the timer goes off, if you're still in the flow of things, then keep going.
Or, if you find yourself stuck, and feel like you've transcribed everything in your head, then set the notepad aside.
Return to your notepad in a day or two, and repeat the exercise, using the ten minute intervals to flesh out your previous scraps and start connecting the dots into a 'story'.
Like any other activity, starting is the hardest part.