Once accepted to join an affiliate program you can log in and access the marketing material that is available. Some affiliate programs have way more than others. Every affiliate program would at least have a link that you could use to track the leads and sales you generate from your effort.
Stats of your performance and earnings are gathered automatically when you use the marketing material found after logging in to the affiliate program.
The rules of an affiliate program and the commission rates (how much you earn for generating sales and leads) is set by the company that owns the affiliate program. The commission rates sometimes are higher for top-performing affiliates.