This is like asking "How do I build a house"? Short answer: You earn money every time you either sell a product or generate a lead for a company or it's product or service. Long answer: Read up on this forum... You have to get accepted by the CPA networks, develop a strategy for the offer you are promoting, test, tweak, test again, tweak again, and then hopefully cash in.
Like samspam said, you need to be more specific. Spend some time reading the forums and formulate more specific questions, and you'll get more help.