While it certainly isn't necessary to have an agent help you shop your novel around for a publisher (many writers publish without one), it can be helpful. Many of the larger publishers won't even look at work from an unagented writer and state this in their guidelines. Many publishers feel that looking at work from a writer who has an agent ensures they will be looking at quality material, based on the belief that an agent will not try and represent an author he or she doesn't feel will make money. Consider looking for representation before you submit your work to a publisher. Use the book "Writer's Digest Guide to Literary Agents" to locate an agent who handles the type of work you do. (See Resources below.)
A guide such as Writer's Market's market listings (see Resources below) is invaluable to getting your first novel published. Getting published depends a great deal on sending your manuscript to the markets that publish the type of work you want to publish. The Writer's Market lists all the major and small book publishers, along with their submission guidelines, what they pay, whether or not they accept material from writers without an agent and what they pay. This is the best way to choose appropriate markets for your novel, increasing your chances for success right away. Online subscriptions are available.
Many first-time writers blow their publication chances by not following submission guidelines or by writing a poor query letter. Keep your query letter short. Introduce yourself, let the editor know the name of your novel and that you would like them to consider publishing it, and give a brief summary of your previous publishing credits, if you have them. Include a brief synopsis of your novel. Wrap the letter up by thanking the editor for his or her time. Always do your research to ensure you have the right name of the editor and that the editor is still at the publishing house. You can get this information from Writer's Digest's market listings. Do not tell the editor how wonderful your novel is, how well you write, how much your friends and family enjoyed your novel or how you know the novel will be a bestseller for them. Let the novel speak for itself.
Many writers choose to forgo the traditional publishing process in favor of self-publishing. Mark Twain, Edgar Allan Poe and T.S. Eliot are examples of famous authors who self-published. There's nothing wrong with publishing your own book, but be aware that a lot of hard work goes into it. You alone are responsible for book production, marketing and other expenses associated with running a business, which is essentially what you will be doing when you publish your own books. There are numerous resources available for the author who would like to self-publish (see Resources below).