This guide form Poets&Writers is a good starting point for learning about literary agents.
Some websites with tips on writing a query letter:
The following books provide information about publishers, tips about what kind of writing they are looking for, where to send submissions, and much more. The first one, Writer's Market, is especially well-know.
Different publishers will have different protocols for accepting submissions.
DON'T just send your manuscript off to an editor's email or the publisher's address, unless instructed to do so in that publisher's submission guidelines.
DO research the publishers you are interested in and make sure you follow their submission guidelines.
Submission guidelines are not always easy to find. Here are some tips to help you navigate publisher's websites!
Investigating The American Directory of Writer's Guidelines may also be helpful, though you'll still need to find the publishers' current website, to make sure the guidelines are still valid.