Whether or not there is an answer to that question is a contradictory circumstance. I can discuss how one can get started, but at the same time, how the hell should I know? The aspiring writer could either have an idea as to what they want to write, or have no idea and still submit a request for guidance. If it's the latter, then the individual either is not a writer or not ready to write. I shouldn't have to help someone find a good idea to write about. If that were the case, I'd write it myself.
Strangely enough it is easy for me to give an aspiring writer an idea of how to begin, but an individual who wants to write a book needs to have the tools readily available, and I'm not just talking about the physical tools; a laptop (or typewriter, or paper and pen), I’m talking about the mental tools—they need to have a focal point; an idea where the story beings, or at least have a grasp of the tale’s plot. When one has serious notions about writing a book, one must have the ground work laid out.
First, consider your genre: What sort of story do you want to tell? Is it Romance, True Crime, Thriller, Horror, etc., etc…
Second, you should have an idea as to who your players are, and they all must have a specific “job” that is integral to the plot and/or moves the plot along. After all, that’s what characters do—they propel the protagonist, and antagonist, forward.
From there it all hinges on spending time with your subject matter and your characters. I’ve written a couple of posts regarding the importance of understanding who your characters are first and foremost (Using Your Characters To Write a Great Story / Developing Your Characters). If you don’t know who your characters are, if you don’t familiarize yourself with them, then it’s a guarantee your book won’t be successful.
You need to understand who is who and how they all fit together as a whole. I personally recommend spending time with your characters (in your head, of course) before you put pen to paper, so to speak, no matter how long that takes you, and that you have a solid handle on which character is being used for which purpose. Things change during the course of your writing; your first draft may change dramatically from the finished draft that you are ready to submit to publishing houses or self-publish, so it’s not as though you need to have your characters grounded and disallow them to budge, but by the time you’re moving things around or changing ideas, you should be well acquainted with the players.
I’ve also touched on the subject of writing what you know (Why You Should Write About What You Know). Basing a novel on a subject that you are familiar with can help you move through your ideas comfortably. If you do this it means one battle will already be won and in your favor. All that will be left is for you to A) Figure out the particulars of the story, and B) What role your characters play in it. That sounds easier said than done, and that’s because it is. Regardless of whether or not you’ve conquered the subject matter because it’s common knowledge to you, and you’ve grown so close to your characters that you send them Christmas cards, you still need to connect the dots. And in addition, you’re still going to need to research any information you’re not familiar with that fits into your story and is necessary.
And speaking of Research...It needs to be done. Research can be the difference between a successful novel and a devastating embarrassment. If you don’t have the wherewithal to research your subject matter, then forget about writing—you weren’t meant to do it. Writers take pride in their work, just like anybody else who loves what they do, and in the case of being a writer there’s the added concern that strangers from around the country, the world even, have access to reading your work if you choose to publish. If you don’t mind looking like an idiot, then by all means…publish drivel. I’d hate to encourage that, but the sad fact is this: There are many self-published authors who should not be, for all intents and purposes, published.
I guess the bottom line is figuring it out is not as simple as asking a writer who has traversed the playing field that is writing to help you collect your thoughts. A true writer will be capable of sussing out much of it all on their own—a true writer doesn’t need help getting started because a true writer will figure out where to begin. How can I tell you what you should write about?