It really all depends on what you want to write about. Writer's Market has some advice and good ideas on where to get started. There is also a magazine called Writer's Digest that has articles about the writing process. If you have a publisher in mind, check out their website for submission guidelines. Most publishers have clear directions for what they will and will not accept for consideration.
If you do not have a publisher in mind, here are just a few basic tips.
-Create a table of contents. This will be a road map for you for writing. When you write a proposal to a publisher, this will be a key element.
-Type in double space, with one space after each period and comma.
-If you are working on nonfiction, keep a careful list of sources and citations. This will be very important in the publication process.
-Find someone whose advice you value to read and critique your work. Choose this person carefully, as the wrong advice can cause you problems and make you doubt yourself.
-Write every single day. Even if you are not working on the book, you are practicing the process of writing and improving as a writer.
-Don't get discouraged. Remember that it often takes many drafts before you hit on good writing!