try something on ghosts. Or if you want sci-fi make it on some flesh-eating alian bacteria that consumes their victims and makes exacts clones of them invading the earth!!!
or maybe something about children, that's always spooky, you know like posessed children or something..
maybe just try a ghost story...
Oh and on how to start it begin with a scene from the past, you know like the origin of the ghost/flesh-eating bacteria/whatever you'll use. (maybe it started in the stone age and killed off the dinosaurs or stone people?)