I’ve just put up a new musing responding to that much asked question:
The short answer is that there is no one way to get an agent. Luck and hard work both play their part. But first you have to figure out whether you’re ready for representation. Don’t even think about pursuing agents until you have a finished novel. And make sure that novel is as good as you can possibly make it. Then make it a whole lot better. Rewrite and rewrite and rewrite and then rewrite some more before you send it to anyone. And, yes, this does apply to you. And yes it applies to non-fiction proposals too. Even though you don’t need a completed book you do need the best proposal you can possibly write.