Need help!

So I'm writing a thriller for an online competition which ends in November. I'm not sure how to set it off or even get started! Any ideas? Here is the basic summary for an overall idea:
There is a Nightmare tearing down Shadowridge. The myth that cloaked the town in fear and Oath of Silence now comes to life. People begin talking, and then, people begin disappearing. Files of missing people cover desk after desk in the Shadowridge Police Department. Families begin to leave town; and soon, it starts to look like the haunted city of dry bones. No bodies to be found, but the death that gripped the city cannot be overturned. Some lives, taken by their own hands, children, tortured and tormented. What is happening?!
_____
"Did you have that nightmare last night too?"
"Don't Margrette. Don't dare say-"
"The Nightmare visited me last night..." Margrette continued more quietly. The horror that struck her friend's face as two black claw-like hands reached out of the darkness of the suite and grasped Margrette in its arms, until she too, disappeared... no trace but her scream that echoed in the lingering shadows.