The two of the more common ways to get a book published are through a literary agent or through direct contact with a publisher. Any other way (self-publishing, vanity presses, etc.) may cost you time and money you probably don’t have.
The first way, via an agent, is probably the best way, since a good agent knows who’s buying stories, what kinds of novels are selling, what constitutes a good contract, and what’s necessary to market your books effectively. Several books are available that can help you find out who the agents are who sell your kind of story. One of the best is The Literary Market Place (available at your friendly local library or online as a pay service), a book listing agents and publishers, what they handle, and what their submission requirements are. Another way to track down good agents is by contacting any published writers you know personally and asking them which agents would be the better agents to approach with the kind of story you have.
Make a list of prospective agents based on whether they handle the type of stories you write, whether they take on unpublished writers, how long they’ve been in business, and whether they’ll share a client list with you. If they charge reading fees, axe them from your list. You should never have to pay to get your work looked at or published. If an agent isn’t a member of the Association of Author’s Representatives (AAR), cross that agent off your list. The AAR has an ethical code that protects both yours and the agent’s interests.
Once you’ve compiled your list, send the prospective agents what they expect in a submission (usually some combination of query letter, synopsis or outline of the entire book, and three sample chapters; aka “a query package”). If they ask for the complete manuscript, send it.
Go ahead and send out query packages to several agents simultaneously; but, if any one of them requests the complete manuscript, that agent should be the only one to whom the complete manuscript is sent. Simultaneous submissions of an entire manuscript are usually frowned upon by publishers and agents alike.
The second way to sell a book is directly to a publisher (if they accept such submissions; some only accept submissions through agents). Make a list of the publishers who publish what you write, check the various market guides or publishers’ web sites, and submit according to the requirements listed. If you aren’t sure, write the publisher and ask for submission guidelines and the name of the editor to mail your submission to. Most publishers will want a query package (described above). When submitting this way, your book or query package goes into what’s known as the “slush pile” along with hundreds of others.
If you’re lucky and a publisher makes you an offer, look for an agent before signing anything. A good agent will be able to negotiate the best possible deal for you. Compile a list of prospective agents, then start at the top of the list and contact each of them. Tell them you have an offer from a publisher. Ask them questions about their agency (e.g., who’ve they represented, how many authors´ stories have they placed with publishers, what their fees are, whether they belong to the AAR, etc.). Keep on going until you find an agent you’d like to represent you.