Horror is one of my favorite genres, but I think it's difficult to write. Ray gave you good advice. Your first step should definitely be to decide the tone, subgenre, etc. A lot of horror films have basic plots, so it's okay in my opinion if you don't have a super compelling story. The key is building tension and having characters the audience can relate to. Also, it's okay to have a message in a horror film, but don't drill it in the audience's head. For example, I saw a film called Megan is Missing today. Good film, but it would have been better if the director didn't over emphasize "be careful who you trust" as a theme. Kind of made the film feel like a combination after school special/torture porn film.