That's such a good question, I should have done that for my coursework! I wrote about something to do with religion and used Jerry Springer: The Opera as my primary source of discussion.
You could really write about anything, that is so so broad. Slashers are probably the most easily accessible subgenre to focus on, but be sure to acknowledge that horror is FAR bigger than slashers. Watch
this TV show, I remember it being quite good and it could help you out when talking about the creator's motivations. It's in a few parts but they all worked when I watched it (admittedly some time ago).
The Vietnam War (as you'll find out watching that) was a major influence on horror films, especially the work of Tom Savini.
Also, look at marketing. The Blair Witch Project suggesting that the characters in the film were genuine missing people is a good example of playing on peoples' emotions.
Then, wrap it up with some awesome analysis and you're golden. I can't help you with that bit because it is totally dependent on what your essay is about but if you're doing A Levels, it shouldn't be a problem...right?
Good luck!