One should always have John Carpenter's The Thing on their list, even if you decide to wait until you are more ready!
I really liked the 70s Hellhouse movie, but don't quite recall the title.
You can also consider watching the first entries in classic horror franchises: Halloween, Nightmare on Elm Street, Saw, etc. and ignore the rest because they are kinda garbage. That would also give you the background you need if you ever wanted to watch Scream.
Killer Clowns from Outer Space is really fun.
Alien and Aliens. The others are fine to garbage.
Predator, while we're looking into the scifi-horror side of things.
I hear Kubrick's The Shining is a classic, but I haven't watched it yet. I've also heard Doctor Sleep is great.
Consider Annihilation. Having a great sound system is essential to get the most out of it. It also strays faaaar from the book, so it won't ruin the book for you.
I'll leave it at that for now.