You may not be sure whether you want to self-publish or go the traditional publishing route. There are some benefits and drawbacks to each.
In traditional publishing, an author retains a literary agent, who pitches the book to acquisitions editors at publishing houses. Once the book is acquired, the author is often paid an advance against royalties to be earned once the book is published. The publishing house assumes the costs of editing, design, manufacturing, ebook conversion, distribution, marketing, publicity, and special sales (into retail outlets not served by the publisher's book distributors). After sales of the book have earned back the author's advance, the author begins to receive a percentage of book sales going forward.
In self-publishing, the author assumes all the costs of developing and creating the book, as well as distribution, marketing and sales. There is no advance, and the author keeps all revenue, which is obviously very appealing to many authors. There are some very well-established authors (J. K. Rowling, for example, or J. A. Konrath) who have expanded from traditional to self-publishing for precisely this reason.
Why, then, would an author choose the traditional publishing process over the self-publishing process? For one thing, having a publisher handle all the work (after the book is written, of course) is very attractive. Traditional publishers have infrastructure that allow them to offer professional services (cover design, editorial), as well as distribution channels to retailers. An author working with a traditional publisher does not have to build relationships in the book supply chain, because the publisher already has them. Many authors have found that, while self-publishing helped them get a good start in the book industry, traditional publishers' distribution networks earn them even more revenue despite the costs involved. This is why E. L. James, for example, allowed Random House to acquire Fifty Shades of Grey. Even with a smaller percentage of the royalty, she is able to earn more revenue because Random House can sell the book into more retailing outlets.
The decision to self-publish or not is ultimately the result of many factors. Increasingly, authors are doing both - publishing traditionally where they can, and self-publishing when it makes sense to do so. In an age where tools are relatively inexpensive, and marketing can be accomplished on the Web, anyone can be a publisher.