Research publishers. Websites, books and magazines have information. Start by looking for companies that publish the style of book you have in mind. Look at authors whose work you like in the same genre and check their publishers. Read the submission guidelines --- if they do not take unsolicited submissions, don't bother. If they don't take email submissions, don't send one.
Write an outline for your novel. Not all the details are necessary, but there must be enough to give the reader an understanding of what it is about. It must be a good read; providing a boring list of bullet points as a sample of your writing is not smart marketing. Some publishers like a shorter synopsis and some want more information.
Write a cover letter, one to two pages length maximum. State who you are and your experience as a writer. State why you think your book would be a good fit in the publisher's catalog. Be honest about the state of the book --- is it just an outline or is there something written. The letter should be addressed personally to the main editor in charge of purchasing new material.
Include the first three chapters of the novel. Unless you are already a published author, an outline is a difficult sell. The publisher has no idea whether you can write or even complete a project the size of a book. A beginning writer is more likely to garner interest by having a finished manuscript to show.
Place your material into a large enough envelope and mail it to the editor. If you would like your material back, send a self-addressed, stamped envelope big enough to hold it. If you want just a reply, send a letter-sized, self-addressed, stamped envelope. If the envelopes are not included and the publisher decides he does not want to use your work, you will probably not hear back from him.
Wait. Some publishers say in their material how long it might take to hear back. If after a few months you have not heard anything, send a followup letter, politely asking if they have received your submission.