I highly recommend you read this article , How to write a horror novel, written by  Loren Davis, Brandywine , Richard, Anonymous ( see all)

1. Get an idea! What good is a horror story without a great idea to get it started! To get these ideas, it's a good idea to try to think of them just as you're starting to drift off to sleep - especially because the dark naturally makes you think of all the scary things that could be in the shadows. Try to think of the things that you wouldn't want coming out at you from those shadows, and then begin to think of what might happen if they did. For example: "Oh JEEZ! I wouldn't want a tall woman with a hideous face, long, tangled dark hair, and a long black dress coming out at me with a butcher knife to make me just like her! That would be horrifying!" could be something you'd be thinking about. 2
2. Take care of the details! This is just good, general advice for any story: you - need - DETAILS! What good are your setting, main characters, minor characters, and biggest plot events if you don't have the details to go with them? Once you've got those answered, you're good to go.
3.  
   Plan it out! It's a good idea to write out what will happen in the story, in point form, before you actually write out your story. You can use the point form notes as a guide; they'll keep you from getting lost or going around in circles. Don't be too elaborate in your point form notes; you just want the big things that are going to happen and the order in which they go.
4.  
   Just write! Put pen to paper and write, write, write! Add extra description (and with a horror story, description is your biggest tool in causing shivers, so add enough to make them start looking in the shadows and wondering if maybe, just maybe, what you write about is possible) and more details. Follow your guidelines so that you don't go in circles and lose the reader. Be sure not to use too much mystery, if you're inclined to; your reader needs to know what's happening in order to be scared, after all.
5.  
   Keep track of your characters! You might put a small child under the bed on the fifth page and then end the story without giving the readers their purpose, and concluding their role (the child under the bed is hiding from a serial killer but fails, and is found later, for example). You may have forgotten about them but your readers haven't! If you don't have a good memory for that sort of thing, it's a good idea to keep a piece of paper with all of your characters on it and cross them off when their portion of the story has finished.
6.  
   Finally - Rewrite! Go through your story, edit it, rewrite what parts look like they could be better said, and then get your friends and acquaintances to read the story too! They are usually better than you at picking up mistakes you've made