You're going to need a clear vision in your head of what you want to run before you get underway with anything.
If your focus is on a monster/s, then you need to have the creature well fleshed-out first of all; descriptions, strengths, backstories, shit that makes it scary and so on. Make it weird, make it disturbing, make it scary. Those are key if the horror of your game stems entirely from the one source (ie. the monster).
Your setting is important too; make sure you know it well and can describe it to your players. Jump on Google and snag some images to help you describe it to people; pictures are great for this. You're gonna need to let players know what sort of characters you want for this RP, too; ordinary folks, high school kids, detectives etc. Give them something to work with.
Finally, the most important thing you need is to have a direction for your RP to go in. Something to drive the plot, keep the momentum going. The story, in other words. Having an ending in mind when you start isn't a bad thing, either; it gives you something to push towards, a dramatic finale to the whole thing.
Hope this helps.
