After you know your market, you start looking for agents in places like Writer’s Market, Literary Marketplace, Guide to Literary Agents and other reference sources, most of which you can find in the library or online. Most all the big publishing houses today require agented submissions. This cuts down on the enormous amount of unsolicited manuscripts for the publisher. They use agents to do the first cut. Remember agents represent certain markets as specified in their submission guidelines, so be sure you look for agents that represent your market.
Agents ask for a query letter. You have to condense everything about your novel down to one page. This is a real art and a real pain for most writers. If interested by your query, they might come back and ask you to send the first three chapters and a synopsis. If still interested after that, they might ask for in the entire manuscript. They may not answer you at all. All of this is usually done electronically now. If they like the book, they may offer you’re a contract to be your agent. Then they start the same process with publishers. The whole process could take a few weeks to two years or more. Non-fiction books are usually sold on proposal. (You don’t have to have the entire book finished but you have to have a solid proposal and impressive credentials to write non-fiction.) Novels are usually sold on completed manuscript.