Tips for writing a horror script

So my class has an assigment to make a 15 to 30 minutes short film and for some reason my group decided to make a horror-suspense story and asked me to be writer, I accepted thinking it was going to be easy. But I really can't get myself to think about anything scary or even suspenseful. I tried watching some of my favorite horror movies for inspiration but I can't still get myself to make a decent story. 
  
Now I need tips to write this thing (I only have 2 weeks to do it). So what does a good horror story needs to have to scare you? or what constitutes a good horror story for you? 
 
Thanks in advance.  

Have a theme?

Don't take it too seriously. Try to make people laugh then bring the BAH moment. Try writing about something that scares you or a scary experience you had in the past. 

" Have a theme? "

Don't replicate anything, The best Horror is fresh Horror.

Zombies. Lots of zombies. And make them run, like, really fast.

The best horror is one that makes the viewer feel incredibly weak/insecure. Resident Evil horror with a bunch of guns and an uber-protagonist = not scary. But then Blair-witch project movies basically undermine the viewer at every turn. Although I'm not sure you'll have the capacity to set that up within a 15-30 min timespan. 

the secret is silence

" @ThePhantomnaut said:

" Have a theme? "

Nope >_> "

Actually, now that I've said zombies, it's just reminded me that I always wanted to make a short about 2 close friends holed up in a room during The Zombie Apocalypse™. One of the people has been bitten, and they both know it. Thing is, they haven't seen another person in more than 6 months, and could very well be the last two people alive. So they try to forget about the bite and have a world-weary discussion, all the while knowing what the outcome will be. 
 
The uninfected person knows that this will probably be the last conversation he'll ever have with another human being before a lonely (and probably short) life of scavenging, fighting and hiding. He wants this moment to last as long as possible, both for selfish and sympathetic reasons. Obviously the doomed friend wants the conversation to last as long as possible too. So it's a talky piece where you know the bitten friend has to turn at some point. The conversation doesn't always have to be about their situation or the state the world is in - their friendship will bring up nice, non-zombie related topics. There are a few ways it could end.

" the secret is silence "

IT HAS TO HAVE SEX!  IF THERE ISN'T SEX THEN IT WON'T BE A HORROR MOVIE!  THE SLUTTY GIRL HAS TO DIE WHILE HAVING SEX!  
 
DO NOT FUCK THIS UP ROSE, OR I SWEAR TO GOD I WILL KILL YOU NONSTOP IN MW2! 

Try and create fear in something that's not scary inherently, or that people aren't really scared of. Psycho made people scared of bathrooms. Jaws made people scared to swim in swimming pools. But setting a horror thing inside an abandoned meat factory with meat hooks, dead pig carcasses, rusty metal, and greasy looking surfaces is just a cop out.

" @ArcLyte said:

" the secret is silence "

No. Silence is only used to build anticipation.    Use sound, the most suspenseful horror moments are when the protagonists can hear something but can't see it.  The trapped in an area versus monster picking off group one by one would be an easy shoot in darkness.  You could even embrace the whole Blair Witch/Paranormal Activity genre to make shooting the thing easy, the actors shoot it themselves.    An easy story would be a camping trip in the woods, sitting around campfire, someone hears something, uneasy laughter, everyone goes to bed then screams they all wake, someone missing and people get picked off one by one as they huddle. "

Set it up so that people won't know what's coming, or so that they don't quite get what's going on until the end. Easier said than done, of course, but that's all I got.

Well, in order to right a horror movie, you have to understand the point of the horror genre. 
 
It's not necessarily to SCARE people anymore.  More often than not, horror movies are used as a form of political or social allegory, and a lot of that is thanks to George Romero.  If you look at something as simple as Friday the 13th, that whole franchise in the beginning was about two main things:  don't pick on those less fortunate than you, and don't fuck before marriage.  Meanwhile, there are some horror flicks that are simply true visions of terror, like Midnight Meat Train. 
 
First, you guys have to decide what KIND of horror movie you want to make.  High gore?  Low gore?  Suspense over horror?  Terror over suspense?  What kind of archetypal characters will you use?  What will offend people?  That's a big thing - horror is MEANT to offend people.  If it doesn't offend someone in some way and make them question the idea of what it is that offends them, then it falls flat on its feet. 
 
Second, are you going for a comedic route?  It's a recent trend in horror movies that has done well (see: Shaun of the Dead, Zombieland, Drag Me To Hell).  It's also helped add a level of freshness to horror movies.  Unfortunately, you've got to have some comedic timing and the ability to write a good joke here and there.  If you can't do that, then I'd stick with the basics of horror:  allegory, blood, and scares. 
 
IN THAT ORDER! 
 
If worse comes to worse...just look at your nightmares or fears.  Horror isn't about gore all the time.  Horror is about things that offend, things that are taboo to talk about, things that are evil.  Use that.

Never show the creature/killer. The human mind will fill in the blanks. Your imagination will always terrify you more than what you see on the screen.

" Never show the creature/killer. The human mind will fill in the blanks. Your imagination will always terrify you more than what you see on the screen. "

Well I suggest thinking about what kind of suspense or terror you want the audience to feel. For instance if you have a killer, do you want the audience to know who it is or do you want it to be a mystery. If you let the audience know who it is then you can have some interesting situations where the killer is interacting normally with other people and leave the audience wondering who or when the killer is going to strike. Or if you go with not letting the audience know then you can get most of your suspense from leaving the audience completely in the dark.

I'm thinking campy horror with zombies or huanting of some kind. The trick is to build up suspense so when you got that cheap cmaera trick going off, the audience will buy it. AFterall, classics like psycho isn't scary when you just watch that one scene out of context.

MAKE IT SCARY

Dog with shifty eyes.

I love horror movies that have living humans as the villains because I like it when the protaganists have some hope of victory as slim as that may be.The psychopath is a commonly used villain but also a much abused one in the horror. The psychopath is the easiest human villain a person can write, seeing how they have no motives for their actions other than the joy of killing. The common reason for using this sort of villain is that you want to make a badass that revels in the wet works. The problem is that it's hard to make the psychopath believable and avoiding being cheesy, so watch your dialogue