Asked plainly, this question usually gets one of two answers: "start writing" or "make an outline". Both are wrong, and being wrong about it is the most common reason a book idea sits untouched for years.
The actual first step
The first step is to decide, in one sentence, who the book is for and what it leaves them able to do. Everything else — outline, contents, title, schedule, format — is downstream of that sentence, and none of it can be settled until it is.
The sentence has a fixed shape and it is worth writing it out literally:
This book is for [a specific person in a specific situation], who is stuck on [a specific problem], and by the end they will be able to [do a specific thing].
Why this and not an outline
An outline written before the reader is defined is a map of the author's knowledge. That is a genuinely different document from a map of the reader's journey, and it is the reason so many first drafts open with three chapters of background that nobody asked for.
Once the sentence exists, the outline almost writes itself, because you have a test for every candidate chapter: does it move this reader from where they are to what they will be able to do? Chapters that fail the test are not bad chapters. They are chapters for a different book, and you can put them on a list for later without guilt.
Narrow feels wrong and is right
Every first-time author resists narrowing, because narrowing appears to shrink the audience. Two things are worth holding against that instinct.
First, a broad book cannot be written well. If the reader might be a student, a retiree, a professional or a hobbyist, then every example has to be hedged, every assumption spelled out, and the prose goes flat. Specificity is what lets you write the sentence that makes a reader think this was written for me.
Second, recommendation is how non-fiction actually spreads. Nobody recommends a book by saying it covers a lot of ground. They recommend it by saying "if you are dealing with X, read this". A narrow book gives them that sentence.
Three tests before you go further
The stranger test
Say your sentence to someone outside your field. If they can repeat it back accurately, it is specific enough. If they say "so it is about business?" you have not finished narrowing.
The shelf test
Find the three books nearest to yours that already exist and read their contents pages. If your contents page is a subset of one of theirs, the book has no reason to exist yet. That is a solvable problem — usually by narrowing the reader further, or by owning a method rather than a topic — but it is better solved now. Library catalogues are the least commercially distorted way to survey what exists; the Library of Congress catalogue is public and searchable.
The scope test
How long does the book need to be to deliver the promise? Answer honestly, then check whether you can write that in the time you actually have. Most stalled manuscripts are not failures of will. They are books scoped for a sabbatical being written in evenings.
What about the writing itself?
You should still write in the meantime. Write badly, write out of order, write the chapter you already know cold. What you should not do is confuse that activity with progress on the book — until the sentence is settled, you are gathering material, not building. Both are useful; only one of them ends.
When the sentence will not come
If you genuinely cannot finish the sentence after a few sittings, the usual cause is that you have two books. A method book and a memoir. A field guide and a manifesto. Splitting them is nearly always the answer, and the one you can finish first is nearly always the smaller one.
The other common cause is that the book is really an article. That is not a failure. A well-made long article that finds its audience is a better outcome than an unfinished book, and it is often how the eventual book finds its shape. The writing-articles hub covers the shorter form.
Next
With the sentence settled, move to the full sequence in seven steps, or straight to building the chapter shell. For questions of clarity and register as you draft, the Purdue OWL's material on the writing process is free, thorough and aimed at exactly this stage.