The single biggest mistake I see aspiring authors make is to sit down and write a book for an audience without reading a single book written for that audience in the last decade. See my post, Novice Writing Mistake #5: Not Reading Enough.
The first thing you have to do is determine the audience you're writing for: Picture Books (PBs) and Board Books (BBs) are for babies on up through kindergarteners, Chapter Books (CBs) are for beginning elementary students, Middle Grade (MG) ages 8-12, Young Adult (YA) ages 12-18, and Adult, 18+.
They each have their own expectations regarding structure and format (word counts, mainly). PBs run about 500 words or so, CBs 4-10K, generally with a 10-chapter format, MG 45-65K, YA 55-75K and Adult the sky's the limit, but debut authors should probably stick to under 100K.
Then read, copiously, books written for that audience, but especially debut authors' books. After you've determined the genre you want to write, read books in that genre written for that audience. You'll learn the current expectations for that genre and audience. Your MS needs to reflect them if you want a shot at getting a literary agent and traditional Big 5 (soon to be Big 4, possibly) publish.
Your reading can stray outside your target audience, and in fact, I encourage it. I discovered Martha Wells' Murderbot Diaries series in 2020. It's all adult and I don't write adult science-fiction, but I have to feed my adult brain, too.
In my Goodreads lists of books from last year, you'll find a few adult books sprinkled in. Not many, and there will be more this year as I revise and get feedback from my fellow Guppies on my adult amateur sleuth MS.
Good writing involves reading, a lot. Don't let anyone tell you otherwise.
