I’d normally not answer this kind of question because I’m not a horror writer, but I see that, so far, you’ve gotten few answers so I’ll say a couple thing.

1. Mystery and horror are, more than most genres, driven by specific guidelines. Find a book/class/mentor who can teach you the correct way to write horror— the craft of it— and follow their advice to the letter. Follow it absolutely even if you think it’s silly, because that’s what will get people reading what you write. (Maybe this is the place to mention that when I started writing, I didn’t sell until I ran across an article in a writing magazine that laid out the specific steps to writing a short story, after which I started selling immediately.) Even if you self-publish, readers subconsciously, like editors consciously, recognize whether a piece is written correctly or not.
2. Remember that the most important aspect of horror is suspense. Learn how to generate suspense and then do it. Overdo it. Overdo the overdoing.