Before you even take your first step toward getting published, understand how publishing works. An agent representing a writer submits a manuscript, or a book proposal, to editors at publishing houses. If the editor likes the book and gets permission from the publisher to acquire the book, the editor makes the agent an offer. If the writer accepts the offer, a contract is signed and the author is responsible for delivering a manuscript. The editor and the writer work on the manuscript together until it is acceptable to the publisher. The publisher then prints and distributes the book.
Your first step is to finish your manuscript if it's a novel. It is rare that an unfinished novel is accepted by a publisher from a first-time, or even experienced, writer. Fiction is not a genre bought on spec. A work of non-fiction is different. For non-fiction, write a proposal consisting of one or two sample chapters, a chapter outline, market research and biographical information. Once you've written your novel or proposal, go back and rewrite it until you've made is as good as it can possibly be.
Now it's time to get an agent. It is hard to get a contract with a publisher without agent representation. Some publishers won't even read manuscripts or open query letters that don't come from an agent. But there are publishing houses that do consider material that doesn't come from an agent..
You should find an agent who handles projects similar to yours. Some agents specialize in romance novels. Others will only take clients who write historical fiction. Read Writer's Market and Writer's Digest or visit Agent Query (agentquery.com) for information about agents and their individual submission requirements. Once you've identified potential agents, write a personalized query letter to each outlining your project. A response may take anywhere from a week to a year, depending on the agent.
Your agent may work with you preparing your manuscript for submission to publishers. While manuscript edits are always negotiable, be aware that your agent is suggesting changes to make the book more marketable.
If the publishing house likes your manuscript and offers you a contract, work with your agent and editor to fashion a contract you're satisfied with. Once the contract is signed you'll either write the book, if it's non-fiction, or, if a novel, begin editing the book with the publisher. Expect it to take a year from the time a contract is signed before the book is published.
Working closely with your editor before the book is published is a crucial part of having your book be a success. Be open to suggestions from your editor and treat the editing process as what it is: a creative collaboration to produce a top quality book. Finally, you and the editor will work with the sales and marketing departments at the publisher to make sure the book gathers attention and is distributed widely so it sells as many copies as possible.