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. If you are contacting an agent for the first time, you should include return postage and an envelope. It's okay to query several agents at once, but never make simultaneous submissions to more than one agent. Like most things, that depends. Four to five weeks is the norm, but there are plenty of reasons why this norm gets violated.