Make the sentences vivid. Don't just say 'It was scary.' Instead, say something like 'The walls seemed to ooze a dark liquid that smelled of decay, and the air was filled with a deathly silence.' Vivid descriptions make the horror more palpable to the reader.
One idea could be a haunted house where every night at midnight, the walls start to bleed. The people who enter can hear the screams of the previous owners who died mysteriously in the house. Another idea is a cursed doll that comes to life and follows a young girl everywhere, causing strange and terrifying things to happen around her. Also, a story about a group of hikers who get lost in the woods and are hunted by a supernatural creature that can blend in with the trees.
There was a story of a strict teacher in a boarding school. He was known for his harsh punishments. One day, a group of students played a prank on him. After that, strange things started to happen. Their books would disappear, and they would hear his angry voice in the empty hallways at night. It turned out that the teacher had a heart attack due to the prank and his spirit was angry at the students.
The moon was a pale, sickly orb, casting long, distorted shadows that seemed to writhe like living things. The wind howled through the bare branches of the ancient trees, sounding like the wails of the damned.
One more for you. 'She walked into the empty basement. A strange whisper told her she would never leave.' The basement is often a place associated with the unknown and danger in horror stories. The sudden whisper indicating that she's trapped is very scary.
First sentence: I woke up in the middle of the night to a cold draft. Second sentence: As I reached for my blanket, a hand colder than ice grabbed my wrist from under the bed.
One opening sentence could be 'In the dead of night, a strange scratching sound came from the attic.' A possible complete story: In the dead of night, a strange scratching sound came from the attic. I lived alone in that old house. Ignoring it at first, I tried to go back to sleep. But the scratching grew louder. I slowly made my way up the creaky stairs. When I opened the attic door, a pair of glowing eyes stared at me from the darkness. I quickly turned on the light, but there was nothing there. As I turned to leave, a cold hand grabbed my shoulder. I screamed but no one could hear me in that isolated place.