The Evil Dead, Hellraiser.
Those are the first two I remember seeing though NOES 1 came shortly after.
Pretty mainstream introduction I know, but, it worked for me.
All three of these scared the shit outta me as a kid...

I remember some movies from Creature Feature when I was young, but the flick that stays with me is the made for TV "Trilogy Of Terror". Scared the crap out of me as a kid.

I remember complaining as a kid after watching The Shining that it wasnt scary (as a kid i didnt understand what the hell was going on). So after the movie my father put on Phantasm II, scared me senseless. Since then ive always viewed the "Tall Man" as one of the scariest horror figures. The next week he showed me Halloween 4 and TCM original. Guess i really havent been the same since.

I never really got scared while watching the film as a kid but could never sleep that night because I'd be thinking about it :D

I'd always be nervous before I threw a tape wondering if I'd get freaked out. I was honestly more scared about my parents seeing what I was watching in case they wouldn't let me watch horror flicks anymore.

The Spiral Staircase-watched this while babysitting one night when I was 14 and it absolutely terrified me, Still gives me shivers now.
Oh and The Evil Dead-remember watching this on video when videos were a new thing, it didn't scare me, but I loved it.

I remember watching The Shining when I was working Nights as a Newly qualified nurse when the old asylum was still open. The care assistant with me had had to go to another ward to help them and I was on my own. I was so scared, I made one of the patients accompany me to the toilet, as it was down a long corridor. That has stayed with me. I love the film but can only watch it with the lights on even now!:D

The movie that really got me into horror was Hellraiser. As a kid seeing Frank walk around without skin and the image of the Cenobites scared the shit out of me. It was one of the first horror movies I saw and it is still one of my favorites.

It's interesting to me to see how many say their seminal experiences were with Freddy & Jason... no wonder we get so many Freddy & Jason threads.

When I was a kid there were no DVD players or even VCRs... so, to watch a movie you either had to go see it at a theatre or see it on TV. Fortunately, there were such things as neighborhood theatres, and the one in my neighborhood had Saturday afternoon "kiddie matinees." Only- they didn't show crappy family movies. Mostly they played horror movies- the Hammer films and all the Corman & AIP stuff. On Saturday night there would be the local creature feature on TV with a horror host- and they showed all the classics, and by classics I mean the real classics- the Universal canon as well as RKO stuff and more.

That's the stuff that taught me what horror was.

80's kids + commercial horror.

To this day I have no idea why, as my dad is anything but a horror fan...but when I was a kid (IE - 4-5ish), he bought me VHS tapes of Dracula (1931) and Frankenstein Meets the Wolf Man. Still two of my favourite movies, and created a love for all things Universal.

A few years later I got into TV shows like Goosebumps and Are You Afraid of the Dark - thats what really sealed me in as a horror fan, I think.