kane

If it is fiction you basically have to write the book and at least get a first draft done. Then you approach agents and get one of them to sign and represent you. That agent then finds the publisher for you. They know who is looking for what so they can help get you placed with a good publisher. The publisher then works with you to finalize the book (your agent may have been doing the same earlier). This means rewrites and edits to make it as good as possible. When the book is done they publish and promote it and you will often end up going out and doing readings, appearances, interviews and anything else they can get for you.

Getting the agent can be the hardest part. They make their living off of selling your work so if they read it and don't think they can sell it they reject you. You could write a kick ass book, but if they don't think they can sell it, they won't sign you. Other agents might sign you anyway because you have talent then they will help you write something they can sell.

If the book is non-fiction you can sometimes research who publishes books like the one you want to write and approach them directly. Since non-fiction has a much more definable audience they will often work with authors directly instead of agents. Obviously if you are writing some kind of major non-fiction book that could be a huge seller then you would want an agent to help you get the best deal possible.