How to Generate Ideas
What kind of idea do you have?
Fiction vs Non-Fiction: You probably already know this, but fiction is a story that you made up completely, and non-fiction is something that is true and factual. (Sidenote: How dumb is it that we categorize books as "not real" and "not not real"???) There are a few things that can blur those lines (roman a clef fiction, creative non-fiction) but generally speaking, you're going to have to pick one.
Age Category: Oldest to youngest, the age categories are Adult, New Adult, Young Adult, Middle Grade, Children's.
The adult market is the general market. Books written for anyone over 18 as the main audience. There is no restriction on content (language, sex, violence, etc), and there are no caveats or restrictions on style and voice. This market takes up the bulk of your bookstore and is not designated as "adult"; it simply doesn't have an age category assigned to it. It is the default setting.
New Adult was a hotly contested issue in the 2010s, and seems to mostly be settled as "not really a thing, but some people will still insist they write New Adult". Books in this category are aiming at the coming-of-age adult market. College years, fresh out of high school, first crappy jobs and crappier apartments. This market found a niche in romance, where authors and publishers felt more comfortable having graphic sex on the page with characters who were over 18 but still had young, fresh voices that didn't sell well or fit in with the adult market.
Young Adult is aimed at readers between 13 - 18 years of age, and often is split it into lower YA (13-15) and upper YA (16+). YA sometimes has "crossover appeal" which means the characters are right on the edge of not being teens and the story appeals to adults, too. Depending on how old your target readership is, there can be a lot of opinions about language, sex, and violence on the page. Most (not all) YA books will "fade to black" instead of putting explicit sex on the page, and the violence will tend to be quick and/or less graphic (but again, not always). This is the market that exploded when Stephenie Meyer wrote TWILIGHT and then quickly endorsed Suzanne Collins' HUNGER GAMES (though it existed before that and exists well outside the science/fiction fantasy of the late 00s). Your general word counts in Young Adult will likely be less than their adult counterparts. For example, Adult Fantasy can easily sit at 100k words, but YA Fantasy can sell at 70k.
Middle Grade is aimed at readers between the ages of 8 - 13. The "tween" years. These books are heftier than chapter books, sometimes as thick as YA books, and I have absolutely no idea how to write any of this. People who write MG well are wizards who deserve all kinds of money for the magic they work.
Children's books can be broken down even further, and are usually based on reading level. Chapter books, emerging readers, picture books, board books, etc. I also have no idea how to write any of this and will refer you out to other experts.
Fiction Genres: I've taught an entire class on fiction genre and sub-genre, and even that didn't dive deep into anything, so this is definitely a very broad subject. Here are some VERY general fiction genre definitions. When giving word counts, I am referring to general word counts for a debut or mid-list author. There are exceptions in every genre and every category, and you probably know them because they are runaway bestsellers. JK Rowling debuted with a 75k MG fantasy, but you are not going to write the bestselling series of all time. Stick to the conventions until your agent and editor agree to turn you loose on your 400k magnum opus.
Literary - General fiction, often more character-driven (instead of being plot-driven). Focuses on prose and structure, often tackling big subjects or heavy feelings (though not always). "Classic" literature and anyone who is striving to write "The Great American Novel" will likely fall into this genre. General word counts: 80-110k. Popular examples: INFINITE JEST by David Foster Wallace, LIFE OF PI by Yann Martel, A THOUSAND SPLENDID SUNS by Khaled Hosseini.
Upmarket - Like literary fiction, except more commercial. Sometimes called "commercial fiction" or "book club fiction", this is fiction that is literary in feel but a little more easily accessible to the mass market. These books get turned into movies more often than other genres (I think, based on a casual observation of the market). General word counts: 70-100k. Popular Examples: GONE GIRL by Gillian Flynn, ATONEMENT by Ian McEwan.
Women's Fiction - Literary or upmarket fiction, except the main character is a woman and the author is often a woman. It is utter bullshit that this is a separate genre, but it is what it is and I am not in charge of this decision. General word counts: 70-110k. Popular Examples: LITTLE FIRES EVERYWHERE by Celeste Ng, anything Jodi Picoult writes.
Historical - Any literary, upmarket, or women's fiction that is set in a specific time period in the past (usually pre-1990, but that's not set in stone) and the time period has significant influence on the characters or the plot. General word counts: 70-110k. Popular examples: THE HELP by Kathryn Stockett, THE OTHER BOLEYN GIRL by Philippa Gregory.
Romance - A novel in which two people meet and fall in love. Must end in a Happily Ever After or Happy For Now. The amount of sex in the story has no bearing on its classification as a romance. Romance novels can be entirely G-rated with nothing more than chaste kisses or hand-holding, they can have fade-to-black love scenes, or they can have graphic, explicit sex on the page over and over again. General word counts: 55-90k. Romance sells more than any other genre, and has more subgenres than any other genre. If you can dream of it, there's a subgenre for romance: paranormal, cowboy/western, historical, fantasy, erotic, sweet/clean, LGBTQIA+, young adult, contemporary, suspense, religious/spiritual. WIthin the subgenres, there are specific sub-subgenres with their own superstars: Regency romance, urban fantasy paranormal romance, historical young adult romance... the list really goes on and on. If you want to write it, there's a market for it, I promise.
Fantasy - Any story that contains magic is a fantasy. The magic doesn't have to be central to the plot, but it does have to be present (for the most part - there is the rare second-world, non-magic fantasy that really feels like a historical not set in our world, but those really are rare). General word counts: 75-125k, with notable bestsellers in the 125-400k ranges AFTER they had proven sales records with smaller books first. Fantasy also has a lot of subgenres: Urban Fantasy that is set in a city or at least a town, usually with supernatural creatures like vampires and werewolves; High Fantasy in which the magic system is complex and integral to the story; Low Fantasy in which the magic system is secondary or non-essential; Epic Fantasy in which the fate of the world is in the hands of our protagonist and the story spans several books. Other subgenres and sub-subgenres exist, and these often blend with each other or blend with other genres, like historical or romance. Popular examples: GAME OF THRONES by George RR Martin, MISTBORN by Brandon Sanderson, HARRY POTTER by JK Rowling.
Magical Realism - Fiction that originates with Latinx storytelling elements in which the fantastical is considered entirely mundane but is a metaphor or allegory for something in our real world. General word counts: 80-110k. Popular examples: Anything by Gabriel Garcia Marquez.
Science Fiction - Any story that explores the possibilities of technology that we currently do not have at our disposal. Science Fiction and Fantasy often get lumped together and the two share some bones, though outwardly they look quite different. What fantasy accomplishes with magic, science fiction accomplishes with technology. Dystopia is often categorized as science fiction, due to its near-future setting. General word counts: 75-125k. Science fiction doesn't tend to have the mega-door-stoppers like fantasy does, but it happens. Popular examples: Anything by Jon Scalzi, or I, ROBOT by Isaac Asimov.
Horror - Fiction that is created to elicit a physiological response in the reader. These can range from psychological horror, in which there is a lot of suspense but not a lot of blood on the page, to supernatural monsters, to bloody and graphic stories of extraordinary violence. General word counts: 70-90k. Popular examples: almost anything by Stephen King, I AM NOT A SERIAL KILLER by Dan Wells.
Non-Fiction Genres: There is a lot of nuance within the non-fiction world, and non-fiction outsells fiction almost at every turn. In order to sell non-fiction, you need a platform. That platform can be as simple as your educational and professional credentials, or it can be as complex as a blend of your social media following, blog/website output, education, professional credentials, and more. These are some of the most basic genre classifications, though every single one of these will have lots of sub-genres and sub-subgenres.
Memoir - A narrative non-fiction that tells the story of a person, group of persons, or a major life event. This could be very narrow, like Elizabeth Smart telling the story of how she learned to forgive her kidnapper/rapist. This could be very broad and long-reaching, like Trevor Noah telling the entirety of how he came to be the person he is, starting before his birth with his mother's wish to become a mother and leading all the way up into his adult life and early career. Celebrity memoirs sell well, sensational memoirs like Tara Westover's story about re-educating herself after a cloistered homeschooled childhood also sell well, and there is always at least one memoir on the bestseller lists about a woman rediscovering herself after divorce, like EAT PRAY LOVE by Elizabeth Gilbert. Memoir is treated like fiction; you have to complete the book, edit it, polish it, and query it like fiction. General word counts: 60-90k.
Personal Essays - Like a memoir, but told in non-chronological snapshots. These are often humorous or themed in some other way. Tina Fey and David Sedaris have both written personal essay collections.
Biography - the life story of another person. Usually they need to be famous or noteworthy. Word counts are all over the place and depend greatly on your voice, your age category, and the popularity of the person you're writing about. Walt Disney and George Washington get 250k doorstoppers; someone less well-known and less well-documented might not command that kind of heft.
History - Tells the story of a real, historical event. Sometimes this gets lumped in with Biography, like a story of Thomas Jefferson and the Tripoli Pirates, but that's really just one series of events, and it includes much more about other men than Jefferson.
Self-Help - This is a style of prescriptive non-fiction that sets out to help the reader be better at something. Sometimes it's to be a better version of themselves (You need a big-ass platform to sell one of these), sometimes it's Marie Kondo helping you organize your house. These can be science-backed and data-driven, or they can be inspirational and conversational in nature.
Research-based Non-Fiction - Any non-fiction that is (surprise!) research-based. Alfie Kohn's books on our school systems, Freakonomics, and a whole lot of the diet/wellness industry counts as research-based non-fiction.
Pop Science - This is research-based non-fiction with less emphasis on the research in the text itself. Very occasionally, these books might be inexpert ramblings, but usually not. These often have very zingy titles, like GEEKS SHALL INHERIT THE EARTH or HOW TO BE A HEPBURN IN A HILTON WORLD.