Writing books is no easy task, but if you have the passion, then don't be afraid. You can do this.
Step one: Idea/Premise/Logline/Comp Titles
That beautiful idea rattling around in your head. "What if X?" Take that question and go a little further. Expand to two sentences (they can be bad sentences) to cover your main character and their needs, the conflict, the stakes, and the resolution (what changes).
The logline will be a cleaned up one sentence version of your premise that usually ends on the "if (the character) can't do X, then (consequence)." This is handy for when someone asks "What's you book about?"
As for comparison titles, finds books or films that are relatively well known and fit the vibe/general story of your book. For example: Gilded Lies is X-Men meets Sense8, but not many people know Sense8, so I say it's LGBTQ+ X-Men.
Step two: Blurb/Pitch/Synopsis
First it helps to identify the tropes you plan to use in your story. I'm not kidding. You'll thank me later for marketing/pitching.
Take your premise and go deeper. Develop a 200 word exciting description of your story. Start with the character, get into what they need, where they are, what their problem is, how they think it needs to be fixed, the obstacle, and then the consequence of failure.
I know it sounds like a lot. Make some lists and tie a few ideas together. Save your various versions to tease out the best lines later. Try over and over, get a friend to look at it, let it rest, read it aloud, and then do it all again.
Step three: Character Development/Plotting/Outline
Characters. I recommend GMC by Deborah Dixon. K.M Weiland is another source of writing craft books I respect. Character development is complicated but it often comes down to two basics: what do they want versus what do they fear. Those two aspects should stand in opposition to guide your character's growth.
An excellent book on plotting is Story Engineering by Larry Brooks. It's a specific breakdown of books by events and percentages. Excellent for an analytical mind.
There are dozens of books to consider for plotting, and you'll probably read several before it clicks. Don't shy away from putting in the work to understand this key story element. It will show if you haven't.
Outlining. This can be as simple or complex as you like. I reference Story Engineering for all of my outlining to help me build the bones. Sometimes I write just a nine-sentence guide. Other times I'll write a 30k outline with details for every scene (or at least every chapter).
Consider yourself a pantser? Cool! Your brain houses the planning for you, but make sure and read those structure books so you don't get lost.
Step four: drafting
The fun part! Sit down and let the words flow. Try not to overthink it. You'll probably edit the hell out of the manuscript after it's complete. Focus on getting it on the page. If you plotted ahead of time, you'll find that to be an invaluable roadmap when you get stuck (everyone gets stuck, get's blocked, gets freaked out by the blank page. It's okay. Promise).
Step five: editing
Step six: editing again
step seven: probably another edit
Find a critique partner for level 1. Betas for level 2. Professional for level 3.
Developmental editing: story and character level editing.
Line editing: line level editing to clarify sentences and maximize impact.
Copy editing: searches for mechanical mistakes similar to a proofreader. (Copy and Line editing may overlap depending on the editor.)
Proofreader: the final pass by someone with fresh eyes and a background in language mechanics for one last look.
Step eight: Query for Trad Publishing/Indie Publishing
Here is where you'll need more specific advice depending on your choice. Hopefully, this list has helped you get started.
For indie check out: Sarra Cannon, David Gaughran, Becca Syme.
For Traditional Publishing: I have no idea. 😅