in Editing
The first page has one job
A first page does not need to explain the world, introduce the cast or hook with a shock. It needs to make the reader turn to page two.
First pages attract more revision than any other part of a book, and more advice. Start with action. Start with dialogue. Never start with the weather. Never start with a dream. Open with a hook. Much of this advice contradicts itself, and much of it confuses symptoms with causes.
The first page has one job: to make the reader want to read the second. Everything else is optional.
What makes a reader continue
Readers keep going when they are curious and when they trust the writer. Curiosity comes from a question, something unresolved or unusual that they want to understand. Trust comes from the prose: a sense that the person writing knows what they are doing and will reward attention.
A first page that achieves both can take almost any form. A quiet domestic scene can do it if something is slightly wrong. A long descriptive paragraph can do it if the voice is compelling. An action scene can fail if the reader has no idea who the people are or why the outcome should matter.
What the first page does not need to do
It does not need to explain the setting. Readers will accept being dropped into a world and pick up context as they go, provided they are given enough to follow what is happening now.
It does not need to introduce the main cast. One character, clearly drawn, is usually better than several sketched quickly.
It does not need to include the inciting incident. That can come later in the chapter, or even later in the book, as long as the reader has a reason to wait for it.
It does not need to be a shock. An opening explosion that turns out to be irrelevant, or a dream sequence that is immediately undone, can feel like a trick. Readers notice.
Common first-page problems
Say a first page opens with a character waking up, looking in a mirror and reflecting on their life. This is often cited as a cliché, but the real problem is usually that nothing is happening and nothing is at stake. The same scene with a question in it, such as a stranger's coat on the chair, might work perfectly well.
Other common issues: too many names in the first few paragraphs, backstory before the reader cares about the present, and prose that is working hard to impress rather than to communicate. Overwritten openings are especially common, because writers revise their first pages so many times that the sentences become ornate.
Revising the opening
A useful exercise is to look at where the story actually starts. Many drafts begin with a page or two of warming up before the real opening. Try starting at the first moment where something changes, or where a question appears, and see whether anything essential was lost.
It can also help to revise the first page last, after the rest of the book is settled. Only then do you know what the opening needs to set up. A first page written before the ending existed is often promising a different book.