Well, start with a spooky idea. For the first sentence, describe something normal but hint at something off. Then, in the second sentence, reveal the horror. It's all about surprise and tension.
Another really scary one could be 'My phone rang at 3 am. The caller ID said it was me.' It plays on the idea of the self being something other or unknown, and calls at that time are always spooky.
You can start with creating a creepy setting. Like, 'The abandoned hospital was silent. A cold draft blew through the empty halls.' Or build tension with, 'I opened the closet door. A shadow lunged at me.'
First, come up with a terrifying idea. Then, in the first sentence, set the stage and hint at the horror. In the second sentence, reveal the full horror in a sudden and unexpected way.