You need to figure out which publisher(s) you will be targeting, and read their specific submissions guidelines very closely. Usually they will have something on the FAQ page of their website. Often it is a very detailed list of requirements for submission, right down to the formatting of your manuscript. Publishing houses are all different.
You might think hard about shopping around for an agent, too--not least because many publishers these days do not take submissions from authors without them. The advice I've heard on how to find them is to go to a bookstore, head to the section for the genre you're writing in, and look through the Acknowledgements pages in the books you find there. Most writers mention the names of their agents on these pages, and you can then look them up and submit to them directly.