Go the traditional route. Write a great book, and query agents and publishers until it's accepted.
Find a small publisher. There are thousands of niche publishers looking for the right books. For instance, if your book takes place in a specific area of the country, look for a publisher who only prints books about that area. Small publishing houses are interested in many different things, among them main characters with certain characteristics, particular time periods in history and unusual sub-genres.
Publish on Demand. If you can't find a small publisher to place your book with, you can try a POD publisher. These publishers may charge a fee to publish your book and may not offer help with editing or marketing your book. Additionally, there's a stigma attached to POD books; bookstores often won't carry them and reviewers won't read them. However, if you're willing to do some of the marketing work yourself, you can make money with a POD book.
Self-publish. You can do this with a publishing service, which will copy and bind your book, or do the job yourself. You will be solely responsible for selling your books, which means that you will have to convince local businesses to offer it to their customers along with other merchandise. Many self-published authors end up with dozens of books in their cars as they drive around trying to find buyers.
Use a vanity publisher. You can get a nicely bound book with cover art and your name on the spine, but you'll have to pay for every man-hour that goes into to making the book. People who use vanity publishers usually only print a few copies that they give away to friends and colleagues to impress them.
Create an e-book. This is a book that exists only as a computer file. The buyer transfers money to the publisher who then authorizes the buyer to download the book to his or her own computer. If you have a short book, particularly if it's a how-to type book, consider offering it as an e-book.