Find the main characters. You'll need to know who the story is about, after all, and you need to figure out which characters aren't as important to the narrative. If you're reading a story with tons of characters, you won't want to note down every single character that appears.
For example: for Harry Potter and the Sorcerer's Stone you would write down Harry Potter, Ron Weasley, Hermione Granger, because they are the main characters. You might even note down Hagrid, Dumbledore, Snape, Quirrell, and Voldemort because they figure importantly in the story.
You wouldn't need to write down Peeves the poltergeist, or Norbert the dragon, because while they are important in their places in the story, they don't influence the main storyline enough to be part of the summary.
A shorter story like "Little Red Riding Hood" is easy because you only have to note down Red Riding Hood, her grandmother, the wolf, and the woodcutter (depending on the version).