How to Write a Horror Story
For good examples, look at both 1984 and "The Fall of the House of Usher."
1984 is a horror story in that it describes a civilization that is horrific, that eliminates individual personality, that runs every aspect of your life, including what you know. "The Fall of the House of Usher" is a horror story because it stirs your emotions. It scares you with frightening, supernatural events.
Start by asking yourself: "What scares me most?" Go beyond things such as "the dark," "heights" or "sharks." Look deep into your expectations. Where do you want to go to college? What do you want to do for a living? What do you expect in terms of a future family? Or, more immediatly, What is your deepest fear walking down the halls at SI? What do you fear might happen in your team sport? on your extra-curricular activity? To someone who works with a computer, such as myself, a crashed hard disk can be more frightening than anything supernatural. Advice: the more "real" your story (thus, the less supernatural), the more truly frightening it will be. Look at the poem "The Unknown Citizen" by W. H. Auden. This is perhaps the most frightening end I can think of. Death in utter anonymity.
Form: You must have the five elements of a good narrative in your story: exposition, conflict, crisis, climax, resolution. You don't have to follow this order, however. Many stories begin in the "middle," when your central character is almost ready for a main confrontation, and then backtrack and tell the exposition in flashback form. (Don't worry about 5¶ form.)