Research agents carefully, check them against those listed in Preditors and Editors, check their web sites for submission requirements. Some will want the first three chapers or 50 pages, some will want a synopsis only.
But pay particular attention to the query letter - it has to be a killer, it's the one shot you have of making the agent excited about your manuscript before he even looks at it. Make it rock. And make it brief.
First paragraph should introduce you and your book -- the title, word length, type of book and which line it is aimed for.
The second paragraph is the most important --it must summarize your book in just a few sentences, like a TV movie blurb or 30-second commercial. What is your book about? What is your theme? What is it that makes your characters different, what makes them and their conflict interesting, what will they learn, how will they be changed by what happens to them? Remember the basic fiction formula: characters plus problem = conflict; conflict plus action leads to resolution and change.
The third paragraph is about you - your writing experience and credentials, prior publishing history, if any (of any kind, including articles, poetry, stories); professional memberships; any other relevant information - expertise that helped you write this book, for instance, or another career...
Backspace is a pro forum I belong to and many of the published authors there have shared their winning query letters -
http://www.bksp.org/upload/showthread.php?t=110 - I'm not sure if you have to be a member to view though. Membership is I think $30 a year.
Amongst the membership are lots of published authors, several agents and editors and various writers at all stages of their careers. I think they offer a free 5 day trial period for membership, but don't quote me.
