All good story beginnings should accomplish four things: Introduce the Character, Introduce the World, Introduce the Themes, and Introduce the Conflict.
Introduce the Character
This is the first time we meet your protagonist. You’re going to want to offer the reader an introduction to their mood, their general way of speaking or thinking, and any facts about this person that we’re going to need to understand the scene you’re opening on. By the end of the first chapter we should know how they feel about the situation they’re in when the story starts, their general moral alignment, and their driving motivations/wants.
The hard part is this: this has to all be woven in subtly. Pausing the action to dump this info into the reader’s lap is both off-putting and boring.
People also like to know how to imagine your protagonist, so try to find a way to sprinkle in clues about the character’s appearance in a way that feels natural and relies on the context of the scene we find them in (avoid the ‘looking in a mirror’ cliché!).
Introduce the World
By the end of the first or second page, your reader should have a pretty clear understanding of the location, time of day, and general tone and appearance of the environment around your protagonist.
We laugh at how cliched “It was a dark and stormy night,” might be, but it’s effective. We know exactly where and when we are in just seven words.
By the end of the first chapter or segment, we should also understand the protagonists’ feelings about where they are, why they’re there, and what they like/dislike about it. If they want to leave this place, we should know why. If they want to stay, we should understand the reasons. We’ll probably also have a general idea of the politics and the way the world they live in is organized in terms of class, sexual equality, ethnicity, religion, education, and where your protagonist’s lived experience intersects with all of that.
Introduce the Themes
What is your book about? And how can you reflect that in the first few paragraphs of the story?
This is a harder element to pin down, so I’ll give you an example: at the start of The Hobbit, we’re given a charming description of a Hobbit Hole, and why it is everything that should be appealing. The Hobbit is a tale about leaving home, homelessness, displacement, being a refugee, and what it means to be without a centre of culture or family. It’s a book about the power of and importance of home and hearth, and starts with a description of Bilbo’s so the readers know what Erebor means to the Dwarves.
Introduce the Conflict
Also known as “The Inciting Incident,” this is that moment in a book when something the protagonist is/does/says/witnesses/flees/confronts/etc. directly affects the outcome of something, and as a result, trips them headlong into the start of the plot.
For example, in The Hunger Games, it’s when Katniss volunteers. In The Hobbit, it is when Bilbo declines to go on the adventure, and Gandalf scratches the rune on the door of Bag End. In A Study in Scarlet, it’s when Watson learns of lodgings available at 221b Baker Street.
Generally, the sooner you can get to the Inciting Incident in the story, the better. But don’t sacrifice setting up the other three elements of a good beginning to get the plot started before the reader knows where they are, and who they are joining along the path of the story.
What About Prologues?
Before you include a prologue, ask yourself two questions:
#1 – Why is the information you’re trying to convey necessary for the story, and why can’t it be imparted anywhere else?
#2 – In what way does this prologue enhance the overall experience the reader has with the story?
If it doesn’t do one of those things, then I would rethink where that information or action can be slipped into the narrative elsewhere, and ditch the prologue. If it does neither of those things, it’s likely not necessary, and you can just start with Chapter One.