You want to start a novel at an exciting moment that involves the main character which will draw the reader into the story to see what will happen next, but you can't give too much information, too soon.
Instead, you give the reader just enough information to understand what's going on.
For example, the main character faces an angry goblin in a dark alley of some major big city.
She can hear cars and a police siren which, unfortunately, is moving away from where she is. Mentally or aloud, she cusses her luck for choosing a job like this.
The goblin knocks her gun out of her hand, and it lands in the sewer drain so she lifts her hands, whispers a spell, and flames shoot of her hands, but the goblin doesn't go down. The injury makes him even angrier.
We now know she's a magic user of some sort, the world is ours or isn't ours by little details, that magical creatures can enter here, and it's her job to stop them, and she is in seriously deep poo because she is now defenseless against a furious goblin.
Later, you'll tell the reader about her role as a Guardian of normal Earth and, later still, about her home on a parallel magic world, but you'll do it in bits and pieces like clues to a puzzle the reader is trying to understand.
Having these clues of the world and trying to understand it is as important a puzzle for the reader as the plot, and it's as enjoyable. Don't cheat the reader by giving away too much.
