In desperate need of assistance in trying to understand how to become part of an affiliate marketing company?

I really want to start making money online but I am still unsure of how to become part of an affiliate marketing company. The one thing that I have been looking into is the type of thing where you sponsor a link for a website and then if that website creates income because of your link, then you receive a percentage of that revenue. PLEASE! If you know anything about this concept/process then please let me know!