The first step to publishing a book is finding a literary agent. A literary agent's job is to negotiate contracts with publishers, as well as offering editorial support. Just as you wouldn't go to court without a lawyer, you don't want to try to publish a manuscript without an agent. An agency must be very selective in the novels they represent, as a lot of work is put into each project, therefore many writers find this a lengthy process, as rejections are high.
Take the unconventional route and contact publishers yourself. If your manuscript is not picked up by a literary agent, there is always the option of sending your novel out to editors without having representation. Although rare that your material will be considered, there are occasions when unsolicited manuscripts are read and bought by publishing houses.
Sometimes it's too difficult to gain representation with a literary agency, or get an editor's attention on your own, so self-publishing maybe an option. Many authors find self-publishing to be a positive experience, as they negotiate the terms themselves, and have more control in the process than they usually would if published with a company.