Find out how the agent wants you to make contact. A query letter? Email? Some want you to go to their websites and fill in the information. Those are the usual ways. I don't know any agent who wants phone calls.
Work hard on the query. Don't try to make it sound overly dramatic; make it sound like you. If you've done extensive research for your book, include that information.
Your query is a sales pitch, but make it honest and realistic. "If you take the writing of To Kill a Mockingbird and combine it with Catcher in the Rye, you'll have an idea of the quality of my book." That's bragging and will probably repel agents.
I suggest you work on a précis or summary statement. I'll write about this in a future blog, but it's what we sometimes call the elevator pitch. If you're in an elevator and have 30 seconds to tell an agent about your book, what would you say?
An agent sells for you
but first you have to sell the agents on you as a client.