The way I find my literary agents is I Google: literary agents that accept YA (young adult) novels.
But there are other ways. You can use Writer's Digest, or you could go to the nearest bookstore and look for books that tell you about agencies and what they accept. I bought one, and I'm sorry to say I don't remember the name of it, but I took it back. I didn't like spending hours looking for an agent that accepts the genres I'm looking for, but also accept YA fiction (all my novels are YA, I am a teenager after all). But that was when I was just getting started with my research, it was sometime around January when I bought it.
Anyway, I prefer to look for agents by Googling.
But you should research agents the way you prefer. It will take a while to find the way you like to research agents and such, once you really get into it you'll figure it out.
By the way, I'm actually sending queries to agents now :) I wish I was writing instead though. Maybe later, I guess.