Answer:
I'm going to assume that you're interested in fiction and in having your book published by a publishing house like Random House, Putnam, etc.
The basic steps are fairly simple. First and foremost, you write a really, really good novel. Today, publishers only buy fiction from first-time authors after the manuscript is complete.
Normally, once the manuscript is finished, you shop around for an agent. Agents are insiders who know everyone in the publishing industry, and know exactly what kind of novel each editor is looking for. When you send in your own novel, it sits in a "slush pile" for about a year before anyone pays attention to it. When an agent sends it in, the editor reads it right away. (Some people skip this step and take their chances with the slush pile.)
Once you have an agent, he or she will send your novel to several editors until someone likes it, and buys it. At that point you'll sign a contract with the publisher. You normally receive some money up front, some when the final manuscript is accepted, and some when the book is actually published. Normally the agent will receive 10% to 20% of your payments, as a commission for selling the book.
You make any changes in the manuscript that the editor requests. Once the book is printed on galleys, you'll also need to proofread it. If you're lucky, you'll be asked to make appearances to promote your book.
You hear lots of scare stories about writing: At any given time, there are 200,000 people in the U.S. writing a first novel, but only about 2,000 first novels published each year. The average first novel takes 18 months to write, and earns $7,500 for the writer. But, there are people who make their living writing books, and you'll never know until you try.