I heard a scratching at the window. When I looked, there was a long, bony finger. It started to draw a symbol in the condensation. I couldn't move to stop it. (The unknown entity outside the window is a classic horror element. The act of it drawing a symbol adds an element of the occult and the inability to move is a common fear in horror situations.)
The doll on the shelf blinked. Its lips moved as if to speak. A chill ran down my spine. I knew it was alive. (This simple story plays on the common fear of inanimate objects coming to life. The idea of a doll, which is often seen as a symbol of innocence, suddenly showing signs of life is quite terrifying.)
I like 'The mirror fogged up. A face that wasn't mine grinned back, then asked me if I liked its new haircut.' This combines the common horror trope of the mirror with a really strange and funny situation. The face asking about its haircut is unexpected and lightens the horror a bit.
A good one is 'I woke up to see a figure at the foot of my bed. It disappeared when I turned on the light.'
I woke up in the middle of the night. There was a strange figure at the foot of my bed. It stared at me with empty eyes. Then it vanished into the darkness.
I was walking in the forest. I heard footsteps behind me. When I looked back, there was no one there, but the footsteps continued getting closer.
I opened the fridge and saw a severed finger among the vegetables.
I looked in the mirror. My reflection smiled and blinked without me doing it.
I opened the attic door. A cold hand grabbed my ankle as I stepped inside.
My phone rang in the middle of the night. It was my number calling me.
One of the scariest is 'My phone took a picture by itself. There was a shadow behind me.' Another is 'The clock stopped at 3:13. Then I heard a knock on my door.'
He found a doll in the attic. When he blinked, the doll's eyes were looking right at him.