I've read sites that recommend the "beginning" to occupy the first 25% of your book.
Starts on the brink of change.
Introduces the protagonist and provides a glimpse of his/her character, goal, and conflict.
Sets up the world of the book.
Shows "before" of the world and the protagonist, what they're like before the story events.
Hints at backstory, or at least indicates there is some relevant backstory.
Initiates the situation of the story.
Shows the inciting incident that starts the plot.
Sets up the major story questions (external, internal, interactional) and probably poses the external story question.
Initiates the external conflict.
Hints at the internal conflict.
Shows the start point of the central relationship.
Ends with the inevitability of change.