Before producing a poster of my own, it would be a wise idea to do some research.
Here are different examples of horror film posters.

The very title of this film,
100 Days of Night, shows the setting in the title, as night time is when a lot of horror films are set (darkness = unknown). This poster has a simplistic two colour scheme of black and red. These two colours cause us to instantly think of danger or blood, which is effective because these are two instantly recognisable common features in Horror films. The Poster shows a man screaming in pain and dark blood splatter, causing us to expect violence in this film. The overall look is very stylised, possibly because it is based on a comic book. Despite this, I think this is a very effective poster.

This poster is for the 1996 film 'Scream'. This poster has a more retro feel to it, as it is in black and white and simply shows a girl's shocked expression. We could feel disturbed as the extreme close up can be considered intrusive. The monochrome colours show that she is very pale, possibly from the shock, and could remind the viewer of a ghost. There is a hand to her mouth, which raises the question, is the hand there because she is shocked or is it someone elses hand? This poster also has much more text than usual, giving us more story. However, the reference to 'Scary Movies' could mean that this film is either a Satire or takes place in a less fictional world. The logo of the film features a sharp knife shape on the 'M', which is representative of the knife murders that take place in the film. The unconclusive picture itself allows the audience to come to their own conclusions and suggests a lot of possibilities.

This poster is for the 2007 horror film 'Shrooms'. The main colour scheme of this poster is dark blue, black and white, which is again a common setting for horror films. The full moon in the background is also associated with the genre, as is the skull shape which it makes with the mushrooms, which suggests they lead to death. The films logo is smuged, which could relate to the effect taking the mushrooms may lead to. Most of these posters have a face in it and a simple colour scheme, two elements I will use in my poster.