Just focus on creating a sudden shock. For example, 'The door was locked. Then a face peered through the keyhole.'
Think of a spooky setting and a quick, unexpected event. Say, 'In the dark forest, a strange howl. Then a pair of red eyes emerged from the bushes. You need to build up a bit of mystery in the first line and then deliver a scare in the second line.
The old mirror in the attic showed a reflection that wasn't mine. Just one glance made my blood run cold.
I woke up in the middle of the night and saw a shadowy figure at the foot of my bed. It just stood there, staring.
I woke up in the middle of the night. There was a figure at the foot of my bed, staring at me with empty eyes.
A girl heard a whisper in the attic. She went up. Silence.
The old house creaked. I turned around. A shadow with no face stared back.
A man woke up in a cold room. The door was locked. A handprint slowly appeared on the window.
Walking in the forest, I felt a cold breath on my neck. I turned around, but only saw the dark trees.
I was home alone. Heard a knock on the door. Opened it. Nobody was there.
Well, one possible 'scary two - line story' could be 'I heard a knock on the door at midnight. When I opened it, no one was there but I felt a cold hand on my shoulder.' Another could be 'The old house was empty. As I walked in, I saw a shadow that moved on its own.' And a third one might be 'She walked through the graveyard. Suddenly, a whisper called her name from an unmarked grave.'
One way is to start with a clear idea or theme. For example, if your theme is 'adventure', begin the first line by introducing the main character and their situation that will lead to an adventure, like 'Jack woke up in a strange forest, not knowing how he got there.' Then, in the next line, add some action or a problem, such as 'He heard a strange growling noise coming from the bushes.' Keep building the plot this way, adding details, conflicts, and new elements as you go along. Each line should move the story forward in some way.