I get this question sometimes. Basically, I had the idea, and needed an agent. A friend of mine is a successful author, and helped me by giving me his agent's name. That was a very nice compliment all in itself.
I drafted a chapter outline, with about a paragraph each. I wrote a whole chapter, and then a cover letter and a "why this book will sell" letter (an agent will tell you how to do all that). Then we shipped it to publishers.
The bottom line is
get an agent. You may need to do some writing first, so you have something to show him/her. I happened to have a website and a few magazine articles, so it was easy for me.
