What and how the agency earns its livelihood is something you should know. Read the agent's contract carefully to see what the agency's charges and commission rates are. Comparative shopping never hurts. Look at the agent's track record. Keep in mind, though, that agents will not talk about how much they've made for other writers, because that information is confidential.
Nobody trains in college to become an agent.
Of the approximately 600 literary agents in the United States, all did something different before becoming an agent. In a word, professionally. Everything you send an agent should look as neat and organized as you can make it. Agents have different policies about what they want to see in introductory submissions. Check the agent's policy before making any submission.