Read plenty of existing fiction to get a feel for the genre and style of story that you enjoy. If you're going to dedicate hundreds of hours to writing a book, it should be something that you enjoy writing about. It will show if this is not the case. Be sure to research some of the bestselling books and try to understand what has made them so successful. Is it the unique storytelling voice of the author, is it the subject matter, is it because the story is filled with unexpected twists?
Hunt for ideas and inspiration. Keep your notepad and pen with you whenever possible --- you never know when you might be struck by a brilliant idea. If you know people who work in a related field to the genre of fiction that you're writing, interview them, mine them for ideas. Sometimes, the best works of fiction are inspired by real people and events.
If you don't already know how to brainstorm then learn how, then take all of the ideas you've gathered and brainstorm them as far as you can.
Turn your ideas into a story arc with a beginning, a middle and an end. Split your ideas and details into these three basic "acts" to form your book's blueprint.
Develop your characters as much as possible before you even begin to write your book. The more you know about them, the more authentically you will be able to make them act and react throughout the story. Make a list of your characters' likes and dislikes and their goals. Give your characters some flaws as this will make them far easier for the reader to associate with; it is very difficult to identify with a character who is flawless because no one is flawless.
Create a compelling protagonist and a problem for him. Your problem could fall into one of these categories: person vs. person, person vs. society, person vs. nature, person vs. God/fate. If the problem is person vs. person, then you will also need to create an antagonist. Standard practice here is to make him the antithesis of your protagonist, with mostly juxtaposed goals and principles.
Create a more detailed time line of events. This can be as vague or as detailed as you choose as even the most basic time line can be a valuable tool for helping you to see the complete end-to-end structure of the story and can cast some light on plot inconsistencies early on.
Choose whether to write your book in first ("I said"), second ("you said") or third person ("he said") perspective. The more frequently used styles are first and third person.
Start writing your book. It's up to you whether you launch straight into the first draft of your story or whether you prefer to write detailed notes first, in essence drafting out the entire story minus the dialogue. Either way is acceptable provided it works for you.
Revise and redraft your book until you have a manuscript that you are happy with and that you are satisfied is free of grammatical and spelling errors or plot inconsistencies.
Send your manuscript to some trusted peers, friends or family to read and offer feedback. If necessary, write a final draft based on their feedback.