A few tips for beginner horror writers.
Unlike films, however, the narrative form really can’t rely on cheap tricks (it’s almost impossible to startle someone [read 'make them jump'] with narrative, the little impact that gore has is reduced even further, atmosphere is a lot harder to develop, etc…), so with narrative you’re going to have to develop something which makes the person sitting in their chair looking at words on a page feel uncomfortable in their immediate environment. If you can’t do that, it’s not horror, it’s action with a horror aesthetic (which is often mistaken for horror, but is different. For an example, think about the Resident Evil films, they’re not scary, they’re action with a horror aesthetic).
Now, you can take my advice as you will. I haven’t written horror in a long time, as writing horror has long since ceased to be an aspiration of mine. But I used to write horror back in the day, I also read from books on theory and from some of the better (and a couple of the worse) authors in the genre as well.
My suggestion is that if you want to scare your reader, there are some things which can help a lot. The first, is make the threatening force in your story something which they feel could easily show up in their own environment. One of the best examples I can think of in this regard is the Mothman Prophecies (although it’s worth noting that it’s author, Mr. Keel is a real UFOlogist, and claims the story is real. Whether it is or not, that kind of treatment really ups the ante because that irrational part of the mind which deals with fear keeps asking “what if it was true?”) The entity you see in that story is capable of appearing anywhere, lurking outside your window, speaking to you out of your bathroom sink, making cryptic phone calls, etc… When you read the story, you begin to feel afraid when you get up at night to go to the bathroom, or look out the window at night.
So set your story at home, for authors without a large backlog of experience, this is good advice regardless of genre. You already know what a home feels like, where creepy things could hide, and what would really send shivers up your own spine. Use that to your advantage.
Some things to avoid are cheap tricks like ultraviolence and gore. While I have been known to go on long rants about the myriad of things that H.P. Lovecraft does wrong, one of the things he does so right that it almost makes up for the many serious flaws in his writing, is that he doesn’t show or tell, he suggests. Violence and gore are at best temporarily disturbing, but the suggestion that even now something evil is lurking around any given corner, that’s the kind of thing which festers.
Also, while there are exceptions, some well worn monsters like zombies, werewolves, and vampires aren’t really all that scary. Pick an entity which is almost more existential in the threat it offers, strange spectral entities, demons, extraterrestrials, or, even better, don’t make it clear at all exactly what is behind the scary shit that’s going down. The less you show your monster, the more you let your readers insert their own fears into your story. Take advantage of that. One other big advantage that these less tangible threats offer, is that they can’t be easily fought against. Feeling powerless is a powerful emotion, it’s absolutely terrifying, and the less your characters are able to fight back (suggesting by extension, that the ability your reader has to fight back would be lesser), the scarier your story will be. Of course, a human “monster” can also be used to great effect, questioning whether evil lies in the person riding next to you on the train, chatting with you in the office, or even sleeping next to you in bed, that shit can get scary.