Whatever scares you most could form the basis of your story. The novel "Carrie" is about a girl with telekinesis, but it also explores growing up and trying to fit in (a recognizable theme). Root your horror in something as identifiable. Your life experiences, given the proper dramatic flair and attention to detail, can leap off the page and into the imagination of the reader. Find the chilling in the every day. If you have an aversion to clowns, Ferris wheels or hospital waiting rooms, your audience might feel that fear, too. Take an unusual turn. Take the path less taken. Write what you know.
In horror, it's easy to fall into the trap of sending your main character off to be maimed or incapacitated. These plot traps are often unconvincing, and the reader simply won't care enough to become emotionally invested. Worse yet, they may abandon your story entirely. Create a compelling protagonist. Consider what hurts him the most and use it to raise the stakes of the narrative. Your characters should be relatable, not just pieces on a chessboard to be pushed around in service of your plot. Keep the parts of your well-oiled narrative machine rolling from one scene to the next. Make your audience feel what your protagonist feels.
Now that your audience cares about your protagonist, you can begin to build terror around him. H.P. Lovecraft's fiction is scary for what it doesn't show: the build up -- the terror on the outskirts. Withhold information -- or the monster -- as long as you can for the maximum effect. But don't cheat the readers at the end. Give them a logical and satisfying conclusion. Explicit imagery may pad your story, but you want to affect your readers emotionally.
Read for inspiration, and more importantly, re-read those scary stories that still entertain you. Study what devices the writer used to effectively convey fear. A steady diet of words from a variety of sources will arm you with the tools you'll need as a writer. Reading will help you to avoid embarrassing clichés, such as "it was all just a dream," or ill-planned twist endings. Though popular, vampires, werewolves, ghosts and goblins should be avoided. If you have a new spin on the mythos, try that instead.
Many memorable horror stories stay with us because they are to some degree about the supernatural intruding upon the mundane world. They elicit a reaction in the reader because it feels as if it could happen. Find a middle ground; one which avoids the overtly quirky and unbelievably outlandish. Horror doesn't have to be large, lumbering monsters and discordantly creaky doors. Scary can be quiet, restless, shadowy and strangely unassuming. Small is sometimes better.