Get an agent. Many of the big publishers won't even look at independent, unsolicited submissions.
Send a query letter first. Describe what the book is about. Tell the publisher why they will be interested in it.
Wait for a response from the publisher. If they like the idea, submit excerpts from the book to give them a broader view of the story.
Wait until the publisher asks for a complete manuscript before submitting one. Proofread the manuscript before sending it.
Look over all edits and corrections the publisher makes when it is returned. If they call for any large significant changes to the story, you may need to make a decision on whether you wish to make such changes to your book.
Read books that are in the genre of what you wrote. See who published these books and then look up how to contact them.
Read periodicals on the publishing industry. They can tell you who's buying what type of books and the trends in the industry.
Talk with or write to other writers. They can give you inside information on publishers.