There's no point in querying an agent about a book for a Young Adult novel, for example, if the agent's focus is non-fiction for adults. With the web and market books it's easy to find out which areas of writing an agent represents.
Have a one or two sentence pitch. You need to grab an agent's attention right away. Which would you read first? Two sentences or three-pages?
To create that short pitch, ask yourself what challenge your main character faces. How does that challenge test the main character? What is the climax? Answer these questions consisely and you can more easily come up with that attention-grabbing pitch.
Of course, be sure to follow instructions for querying. Does the agent want snail mail? E-queries? Sample pages? Attachments or pasted into the email? And be sure to spell the agent's name correctly! (I'm an author, not an agent, but I can't tell you how many times people spell my first name incorrectly--and I notice!)
Query about five agents at a time and have a list of five more for the next round. You may not need a next round, you may need several next rounds. Meanwhile, work on your next manuscript, and good luck!