Think about the kind of impact you want your monster to have. Decide if it is merely killing to survive or does it have something far more sinister in mind.
Come up with a basic idea for a monster that fits your intended effect. If you want something passive, yet menacing, consider a ghost. If you want something that tears its victims apart, it can be a werewolf or some other monstrous predator. You can also come up with a unique monster all your own. Determine its powers, abilities and weaknesses, and write them down for future reference.
Flesh out your monster's motivations. Like any other character, monsters need to have reasons for doing what they do. Perhaps yours is a basically good person cursed to become an abomination at particular times. Or it's an extra-dimensional entity that wants a foothold on Earth. It may simply enjoy watching others suffer (think of Freddy Krueger from the "Nightmare on Elm Street" films). Whatever the reason, it will dictate how your monster behaves in your story.
Develop your monster's history and background: where it came from, how it was created and what it's done in the years leading up to your story. In some cases, your story may detail its creation or birth. But the more you as the writer know about it, the better.
Write a physical description of your monster and draw a picture of it. It will provide good description fodder for your story.
Think about the kind of role your heroes will take in your story. You want your audience to fear for the heroes' safety, so they should be in some kind of peril. How they respond to danger will dictate the course of the story.
Flesh out a personality and background for your heroes: their history, their jobs, where they live and how they come into contact with the monster or other horrors. Stress sympathetic qualities if you can -- you want your audience to identify with them -- and try to make their backgrounds as plausible as possible.
Determine your heroes' connection to the monster. Are they just in its way when it wants lunch, or do they have more tangible links to it? The heroes may need to determine why the monster wants them if they hope to survive its attacks.
Come up with a physical description of your heroes, then write it down so you can refer to it while you write your story. You can draw a picture of them too if you have the right skills.