I would imagine that you would have to remortgage your house to afford good size ad's on MTV.com
As for the way the affiliate system works - you are spot on... There are already affiliate systems setup accross the net. You simply signup (for free) and then browse the list of advertisers, find the ads you like and copy 'n paste the code for it into your website... As simple as that!
All visitors to your site will record a "hit" on the banners displayed, any click throughs will be recorded and any sales from these visitors to the merchants site(s) will be credit to your account at the advertised rate... Some pay per lead (loan firms etc) whilst most pay a % commission.
A good example to have a look around is
www.affiliatewindow.com 