Start your book by reading everything you can find in the category that you want to write in. Your book needs to fit into a category or niche but be different and complementary to other top sellers in that category. This begins with reading as research. Once you have read all of the top books in your category, you should have an idea or passion that becomes the core of your book.
Write your book using excellent writing skills, including correct grammar and punctuation. Remove all unnecessary words and cliches; limit weak verb use (such as overuse of the "be" verb), obscure word usage, adverbs and adjectives; and don't repeat yourself. Also be sure to write actively, not passively, and create powerfully engaging characters. One way to create engaging characters is to do terrible things to your characters so that they can grow and change, as well as reveal the characters motivations. Proof for typographical errors before you send the manuscript.
Research agents who handle the exact type of book you have written. Prepare a cover letter, a query letter, a synopsis or a book proposal exactly as they ask for in their submission guidelines. Include a response envelope. (Some agents now allow you to submit online. The same guidelines apply for formatting, and complying with what is requested by the agent.)
Make copies of your manuscript and send the requested portions to agents. You may send packages to several agents at once as long as they allow for simultaneous submissions in their guidelines; simply inform them in your cover letter. Prepare to wait.
Track your submissions so that you don't send a package to the same agent twice.
Acquire an agent. Sign a contract. Your agent will send your manuscript to the appropriate publishers. Prepare to wait. Make any changes to your manuscript that the agent requests.
Acquire a publisher through your agent. Sign a contract. Comply with the terms of the contract to make revisions and changes as required for the publication of the book. This can take several years.
Sell your book once it arrives in stores by doing book signings. You may need to orchestrate your own book tour since only large-name authors generally receive book tours sponsored by the publisher. Network online, have your own website, be available to fans and tell everyone you know to buy your book and drive sales. Speak on the radio and attend local book events, network conventions and conferences.