Read the preeminent names in the genre to see what kinds of stories appeal to a mass audience.
Consider the type of horror novel best suited to your interests. Write supernatural stories or tales of murder and mayhem if you are a fan of that type of fiction.
Discuss likes and dislikes of horror novels by enthusiastic readers of the genre. You may be surprised to learn how many horror fans with pronounced opinions are out there.
Limit your intake of horror fiction and film watching while you compose your story. Being influenced is one thing while being derivative is something entirely different.
Set the mood with a writing environment that mirrors the fiction you are creating. Setting up candles in a darkened room might be precisely the ticket.
Revise patiently in order to ensure you achieve the effect on the reader you are seeking.
Consult agents that specialize in horror novels. Remember that horror is one of the easier genres to break in to for first time novelists.
Remain persistent through rejection and continue looking for houses that specialize in your type of horror fiction.
Consider e-publishers once all other alternatives have been exhausted. This too is legitimate publication.