As a caveat to the above instructions, I point out that you should only send the first two chapters of your manuscrpt, if it's a novel or non-fiction work, plus a synopsis of what the resst of the manuscript is about. If you get a reply showing some interest, ie requeting the rest of the manuscript then send the rest of it. Nowadays it's considered perfectly permissible to send the chapters off to multiple publishers at once. In the olden days this wasn't considered polite practice but publishers are aware that people aren't prepatred to wait six months to get a reply off one publisher before forwarding it on to the next.

You can always send your full manuscript off to a literary agent, which was what J K Rowling did. Worked for her. . .

Good luck!