Write one sentence that summarizes the plot of your book. This can be more challenging than it sounds, as most plots are complicated and difficult to describe in just a few words. However, finding the perfect sentence to describe your book will help you keep focused on what the story is really about when you begin writing.
Develop your sentence into a full page description. Allow yourself to ramble a bit, as you can change this synopsis later. The most important thing is to get ideas down now so you have a starting point.
Create character pages for your main characters. While you can include physical descriptions for reference when you are writing, it is more important to focus on their personalities. Any story needs strong, believable characters in order to be effective. Decide how many points of view you will be using; some books are told through one character, like Harry Potter, while others use several viewpoints, such as Lord of the Rings. You will need to understand these characters best in order to develop your voice when you start writing.
Research information for your book. For example, you may want to start a document with information about the location of your novel. Consider what your character does for a living and collect notes on what that job might be like. Create a folder on your computer to keep track of the research you do for your book so you can reference it later.
Write an outline for your book. There are many ways to write an outline, and you should choose the one most comfortable for you. One option is to create a timeline in a spreadsheet and list the events that will happen in your book. Another option is to make a set of flashcards, each with a description of a scene. You can make your outline detailed or not, whichever works best for you. Once your outline is complete, you are reading to begin writing your book.