Make sure your novel's finished and edited before you start contacting publishers. The only exception to this rule is if you're working on a piece of non-fiction such as a self-help book, where you submit a proposal of your work instead.
Compile a list of publishers to which you'd like to submit your work and look up their submission guidelines by visiting their website or reading about them in the "Writer's Market" books. Note that most publishers will only look at your work if it's submitted by a literary agent. Look up literary agents by visiting sites such as "agentquery.com," "authoradvance.com" and "querytracker.com." (Resources 1, 2 and 3)
Read over submission guidelines carefully before contacting a publisher or an agent. Start off by writing a query letter and only send samples of your book if the publisher or agent requests it. Depending on how you submit your work and which publishing company you've contacted, it can take months to get a response, so be patient.
Make sure your query letter answers these three questions:
"What does your character want?"
"What's getting in their way or slowing them down?"
"What will they do to get what they want?"
Write the first line of your query letter to intrigue the agent or publisher. Avoid asking a question in the first line.
Keep your query letter under 250 words. Remember, the query letter is essentially a business letter. Get your point across without going into too much detail. Save details such as your major plot points and conflicts for when a publisher asks for a synopsis, or a detailed explanation of the entire plot.
Include sample chapters with your query letter if a publisher asks for it in the submission guidelines. Submit more of your work only after a publisher or agent requests it.