Affiliate Marketing Secrets

I fairly know affiliate marketing is a good source of income online,I want to make it a secondary source of income but know almost nothing about it. What do I need to be in the affiliate marketing business? What is the requirement? Do I need to start a website before I can go into the business of affiliate marketing? What is the income projections? How many success stories do people have. How much money does it take to start? I need a lot of briefings and clarification.