The first rule of getting a book published is to avoid writing a book. That may seem counterintuitive, but the main goal of anyone who wants to publish a book is to land a literary agent before spending years writing something nobody wants to read. If you're reading this because you've already churned out a work of genius, go directly to Step 2.) Otherwise, consider the industry: The literary world is a very closed community, and the people who green-light publication accept books only through very specific channels. Think about it: nobody could ever handle reading the mountains of spew that aspiring authors churn out all the time, so the system has established filters to weed out most of the garbage. You need to learn what the filters are and how to get through them.
Agents
An agent is a separate individual who performs much of this filtering process. You most certainly don't want to send a manuscript directly to a publishing house, because the editors there won't read it. They consider pieces only if they come recommended by an agent. Agents read manuscripts, or ideas for manuscripts known as queries and proposals, and decide whether a project has promise. If it does, the agent signs a contract with the author, promising to use best efforts to get the thing sold to a publishing house, in exchange for around 15 percent of the deal. Editors at publishing houses would much rather deal only with agents who have a good track record of presenting quality ideas, so agents can be very choosy about who they sign. Landing an agent, therefore, is the whole idea of the game. Once you have one of those on your side, she will work incredibly hard to get your idea sold.
Agents, in turn, don't particularly like reading 300-page manuscripts. In fact, they don't like reading much more than a page. So the first step in getting an agent to even pay attention to you is to send him a query letter. A query letter is essentially a short summary of your idea, who you are and why you are qualified to write this project.
Can you proceed without an agent? Don't even try. They are worth the percentage they take: 85 percent of zero is nada, and getting a book deal through an agent is better than that.
Next: Prepare a Proposal