Prepare a business plan of action. As well as knowing the content and themes of your book inside out, prepare the vision you have for the spin-offs that could potentially occur: sequels, prequels, merchandise and films for example. Publishers are interested in great books, and publishers are interested in moving units. The more detailed you can be, the better.
Consult listings of agents and publishers and make a list of all the publishers that publish your genre or style of writing. Do as much research as you can to ascertain where your book fits in the literary world. Once you can explain that succinctly to a publisher, you have a better chance of being picked up.
Find a literary agent. Often, publishing houses will not bother to read unsolicited manuscripts. An agent can help you sell your book to a publisher. Draft cover letters to a number of suitable agents and agencies, and put together an efficient and exciting synopsis of your book. Some agencies require proof of some previous publication before they will represent you, and others may sign you up on the strength of three sample chapters.
Consider self-publishing. If the idea of handing over your manuscript to a publisher or agent doesn't feel natural or right and you would rather retain a closer amount of control on how your book is sold, then publish yourself. This method is a personal choice, but with hard work, clever marketing and a good book, you might be able sell enough copies to eventually get your preferred publishers interested in future books, should you wish then to take that step.
Get published in literary magazines. Before taking that big step with your first full-length book manuscript, it is a good idea to have some previously published material out there. This makes it easier to eventually snare an agent. It also provides a record of faith that other writing industry professionals have previously put in you. This goes a long way, especially if you can link the right agent with the right publisher who both like the kind of writing that appears in an associated literary journal.
Educate yourself about copyright law and intellectual copyright theft. You are about to try and push what could potentially be a very valuable book into the commercial arena. It is vitally important that you don't sign away any rights to the book by a mistake or because you didn't know. Read contracts thoroughly, and don't be afraid to want what you want for you and your book.