banner rotation and advertisement

Hello everybody: sorry for this kind of off topic question. I hope that someone might point me into the right direction. I am operating a web portal and think about advertisement. Unfortunatelly I do not have any idea how to start this. First off all how does the client know how many banner views happend? I mean there must be a system which is proving how often. And second off all how to find the right sponsors fitting to the site. Maybe someone knows a third party company which does this for a fee. Has anybody a good hint? Thank you in advance. Andy