There are 2 ways to publish a book, one is to publish yourself, which means you pay a printer to print your book and do the marketing yourself, and make 100% of the profit after paying the expenses. The other way is to cut a deal with a publisher, in which you will only make a part of the money off it (if it's your first published book, it would probably not be a really great deal) the benefits are that you don't have to pay anybody, the publisher take care of it as well as marketing, editing, design and so on.
There are pros and cons to both approaches, almost all successful books who sold a lot of copies were not self published. To really get it going you need a publisher.
The first step after completing the book is either to try and find an agent (who again will take a cut from your earning, but will make sure that you get a good deal and will introduce you to the right publishers) or send copies of your books to publishers hoping someone would decide to print it.
Here is some more information:
http://www.caderbooks.com/pubfaq.html