I have an agent and development deal with a publisher so I might be able to give you some advice.
1. Research the niche/genre of your book. If it is fiction and you feel it could have a large/wide audience they you will need an agent for sure. If it is non-fiction or something that is geared towards a specific audience then you might be able to approach publishers with out an agent.
2. Real agents don't charge reading fees. If an agent wants to charge you a fee to read your book go somewhere else. Real agents are looking for clients, those that want to charge reading fees are looking to make some money. On that note many agents will suggest you get an editor to help you rewrite the book and make it better. That alone is not a bad thing, but be wary. Some agents have deals with editing companies and they send everyone who submits to them to those companies. If they recommend someone try to find out something about them. You don't want to pay a bunch of money to an editor only to have them not really help you improve the book. Editing should make the book better, not just different.
3. While I haven't used it myself, I know some people who have used a book called Guide to Literary Agents. Here is the Amazon link for the newest edition.
http://www.amazon.com/Guide-Literary...9330418&sr=8-1
It lists a bunch of agents and it also has some great advice about how to go about contacting them.
4. Be prepared for rejection. Most agents will reject you. It doesn't mean you suck, it just means they don't think they can sell your book. They are looking for writers who offer something they feel they can sell. If they read you book and don't think they can sell it, they will reject you. Many of them will give you some advice. Listen to them and take it in.
5. Be prepared for rewrites. If not the agent, most likely the publisher will want at least one rewrite. They will always think it can be better. So if you haven't already, start considering ways to make the book better and get to work on the rewrite. The person that helped me get my agent gave me about 10 pages of notes on things she didn't like about the book and felt I could make better. In the end, of course, she was right and her notes helped me a lot. Criticism might sting, but it is how you grow and get better.
6. Know your market. Everyone is looking for the next something. What mean is agents want to know that you know who the audience for your book is. So make sure you know the niche/genre you are working and know where within that niche/genre you fit. Don't tell them, "It is a wholly original book unlike anything out there today." They don't want that, because that is hard to sell. Instead tell them, "It is a thriller that gives a nod to the classics by Elmore Leonard and James Ellroy, but sinks its teeth into a different vein." Now an agent might say, "Oh, okay I see where this book is and where it could fit in the world." Obviously you would use comparisons for your niche/genre.
7. Be realistic. Agents don't want to hear that you intend to debut on the NY Times best sellers list. They want to know that you have other ideas and plan to one day make a career of this, but that you are a realist and understand it will take a lot of work.
8. Ask other writers. You might contact other writers in your niche and just ask if they have any advice on how to get an agent. Some of them will blow you off, but others will actually respond to you and offer you some advice. Take anything anyone gives you and learn from it. It is a long road, but eventually if the book is good and you have some talent you will find someone good to represent it.
Anyway, I hope that helps some.