Mix 2-6 parts horny teens/college kids + 1 part unfamiliar/isolated location + 1 part mysterious monster/villian + 1-6 parts violent deaths + 1 part twist ending = a scary story.
Or if you want to take the easy way out, search for folk tales and urban legends on the internet, take the premise, alter it a bit, and write from that.
Or here are a few premises:
People on vacation in forest cabin, evil spirit takes control of local animal population/a certain type of animals.
Take a standard horror creature convention: Vampires, werewolf, zombies, ghosts, alien, etc. and write a story featuring that. Ghost terrorizes inhabitants of a new house, Vampire hooker in disguise gets picked up by men then kills them to feed, try to raise a loved one from the dead and it goes horribly wrong.