You've decided you want an agent.
90% of writers don't know the right way to find an agent, so how do you become a part of the minority?
1- Know your market.
Decide where your book fits in. Is it a young adult fantasy or better suited for adults? Nothing wastes your time and money more than querying agents that don't even represent your genre of work.
Read copies of Publishers Weekly to see if your book falls in with any other books that are selling well. Study the trends.
Search amazon.com for similar books (Yes, we know your book is wonderfully unique, but find books that are the closest to yours). Visit the library and book stores and see where your book fits.
Compile a list of 10 titles that compare to yours. Check the dedication or acknowledgements and see if that writer mentions their agent. Search the publishers' sites to find the agent's name for that book.
Now you're close to starting your search for an agent:)