Here's some advice
Visit the websites of major publishers. Some accept unsolicited (unrepresented by an agent) manuscripts. Check to see what their policy on submissions is. Follow their guidelines EXACTLY. If they say no more than a hundred thousand words for a mystery novel, don't send them your 600 page masterwork no matter how good it is.
The first step is usually a query letter. Even if they say they don't accept unagented work, most will still look at a query letter. Address it to the editor in charge of submissions for the line - AND GET A NAME, even if you have to call the publisher. Ask "Can you please tell me who your acquiring editor for cookbooks is?" Don't send the cookbook query to the editor who does romance novels.
BE BRIEF. A query should be no more than a page. Tell them briefly about your book. Do NOT send your manuscript with it.
NEVER make the most obvious amateur mistake: "ABC Mystery is an excellent novel and would also make a nice movie" "I'm available for appearances on talk shows" or "I will accept opening offers for 25,000 dollars." (YES, lots of people do this.)
Editors, if interested, will ask for a partial, usually the first three chapters plus a synopsis. If they like that, they'll ask for the full.
Some editors will take the query plus a partial initially. Again, look at the website.
Manuscripts should be professional looking. USE SPELL CHECK and have someone proof for grammar. White paper, one inch margins all around, double spaced.
The procedure for submitting to agents is the same. Check out agent's websites.
A reputable agent or editor will NEVER, EVER ask you to pay money up front.
The publishing house will promote your book and make sure it gets into the bookstores.
Good luck