Study agents, editors and marketing as thoroughly as you have writing. Agents cannot do anything for new writers that they cannot do for themselves, with a little study and logic. They use the same techniques for you that you can use for yourself, i.e. bundling up a manuscript and sending it to an editor. Often, in fact, writers find their agents by selling their first book themselves, then getting an agent based on the sale. This can be valuable if the agent can obtain a better deal, perhaps by negotiating subsidiary deals that an author might not otherwise consider.
Most agencies are small, with only one or two agents.
You may get a hearing on your script while waiting for them to get through shuffling paper for someone else. You may wait weeks or months for an agent to even look at the manuscript. Do not do business with any agency that does not invite submissions, or at least queries. Submit where your script will be welcome. Before agreeing to let an agent represent you, investigate him or her.