Now you've got your manuscript as near to perfection as you can, you can start finding agents to handle it for you.
Very few publishers accept submission direct from the author. It is mostly done through literary agents.
There are a few very important things to consider.
You must be sure of the genre of your work. Agents will want to know this to enable them, should they accept your work, to present it to the right publishers.
It is recommended that you compare your writing with that of another author. You can then say "Anybody who enjoys .......... would probably like my work." or something similar. This again helps the agent to get your work to appropriate publishers.
But the most important advice I can give you is to visit the website of the agent you are submitting to. You will then find out how to submit to them. Every agent has different requirements. Make a note of what they say and ensure that you follow their instructions to the letter. Don't think that you can submit a standard format to all agents. You can't.
Most will accept your enquiry, along with a synopsis, biographical details of yourself, and the first three chapters. But you must make sure. If you don't follow the rules, your book will land up on their slush pile without even being looked at.
Make the enquiry letter personal. Find the name of the person you are sending your query to. Don't address your letter "Dear sir/madam". Give very brief details of the book, what you consider the market place to be, who would be most likely to read it, and give some details of yourself. Much more detailed advice on this subject is available from Bubblecow .
Your synopsis needs to capture the imagination of the reader. Make it as informative as possible. Tell the whole story, don't offer a cliffhanger ending. Oh, and the synopsis should be single spaced and fit on one page of A4 or similar.
You will probably need to send your query letter, a synopsis and the first three chapters of your book. In most cases you will also have to include a stamped, self-addressed envelope to enable the agent to send you the good (or bad) news.
I would recommend that you submit to five or six agents, and if (and when) you get rejections from one, submit to a new one.