What is a good horror game for someone who has never played one ?

silent hill 1 from ps1 ^^ if you can get it ^^

To my mind, horror is not necessary about fighting or running from monsters. There are a lot of excellent and truly creepy horror adventure games like psychodelic classics Sanitarium, philosophical story about depression The Cat Lady (hard to complain about the lack of gore there), lovecraftian horror Darkness Within (both parts), modern psychodelic masterpiece Fran Bow, pixel adventure The Last Door that scares more than tons of games with modern graphics and cheap jumpscares, sad and visually stunning The Vanishing of Ethan Carter and so on. I guess it's obvios from above that a good horror game is about soaking the creepy atmosphere and exploring the story at my own pace for me, and not about playing hide-and-seek and dying again and again while not being able to look around properly; such things just distract and irritate me.

dead space 1 2 maybe 3, alien isolation, bad rats

SOMA is absolutely amazing, and barely scary, may just a bit... unsettling? The setting is horror, but the game itself is just a really good stealth/adventure game.

Among the Sleep.

Bioshock is a very good game which fits the horror genre pretty well and holds up in the test of time. As someone who typically doesn't like horror in movies or games, it was something I could get into with the action, while still experiencing the horror aspect with the atmosphere, suspense, and environment. Not as hardcore horror as some other games but I feel it's a good introduction into the genre in the very least.

Also the story is magnificent

Played evil within and it's closer to original horror survival. It's better than re5.

dying light has some creepy parts during the nights... not only because of how it looks but also because you usually die in 2 hits :3

dead space 1 is also pretty scary. then you can play ds2 and 3 which are more focused on action than horror.

evil within should be good based on gameplay videos i watched, but i still have to play it.

Claire, or Silence of the Sleep, Lone Survivor are pretty nice horrors - they are sidescrollers, so there won't really be panicattack inducing parts, but the games got the tone really well.
Eldritch gets the Cthulhu-atmosphere really nicely, by dwelving deeper and deeper in caves, fighting/avoiding enemies. Not the scary type of horror, rather the creepy dungeon/mine/tomb raiding, exploring type of horror. Mild but atmospheric.
Fran Bow... psychological horror, point and click. More fucked-up/upsetting than really scary, if you are interested in that type of horror.

Resident Evil series
Amnesia
The Evil Within
Outlast

Even The Forest can be scary if you're into it, it has some weird creatures..
Stalker: Call of Pripyat as well

Dunno is someone already said it but, Alan Wake.

Dungeon Nightmare
Cry of Fear

Alan Wake should be a great start. It has a superb story even if the gameplay is a bid repetive.I also wonder why no one mentioned Telltales Walking Dead 1&2 here. Sadly Silent Hill 2 isn't on Steam, which is a great classic. At least you could grab the excellent Resident Evil (1) HD Remaster.

Mountain

From a guy who played many many horror games I suggest you to start with a light psychological indie horror game like: The Silence of Sleep (PC) or Dark Echo (Mobile)

Amnesia (Medieval/Fantasy Horror)
SOMA (Modern/Technological Horror)
Spooky's Jump Scare Mansion (Budget option)