Purchase "The Writer's Market" by Writers Digest Books, or sign up for access to the website. This resource contains a wealth of information on the publishing process, as well as current market contacts and proper formats for publishers.
Make a list of publishers to approach. Make sure that the publishers accept your genre.
Write a query letter in a format that's accepted by your target publisher. Include the genre, page count, targeted audience and your marketing plans. Response time can vary from a few days to a few months. Include a SASE for mailed query letters.
Prepare materials to send to interested publishers. This will vary by publisher, and is typically the first one to three chapters of a fiction manuscript, or a full proposal for a nonfiction book.
Send out more query letters, unless you encounter a publisher who requires an exclusive reading. If this is the case, ask for a specific time frame in which they will decide about publish your book.
Expect a phone call from an editor at the publishing house if they consider offering you a contract. Publishers typically want to know a bit more about an author before offering a book contract.