Ok. So you have a great idea for a book, but haven’t started writing yet. No problem. If you want your idea to manifest physically, you need to send out query letter’s to reputable “literary agents.”
All you need to do is write a 1 page cover letter called a, “query.” I’ve already gave you a website as your first resource to do this correctly. You don’t need a manuscript for this. However, you should have (try and write) a first chapter ready in case a literary agent shows interest in your book project. They may ask for this at a later time. Landing an agent who believes in your project will get your foot in the door to publishers. So get started on this, ASAP.
Send out a few query letters, but make them standout. You may land your first book deal faster going this route. In fact you may get a publishing company interested who will even give you a six figure book advance without ever even writing the book yet. But you better show them a polished manuscript after signing, or they will take you to court. You can count on it. Good luck.