Making Money From Advertising On Your Web Page: How Does It Work Exactly?

When you own a website , how does the whole advertising thing work? For example, if i started a website that eventualy got tens of millions of hits a month, how would i get companies to advertise on my site? Would i approach them ? Would they approach me? Would they supply me the advertising banners? What sort of money is offered for advertising space?
Basically: How does it all work ?
If anyone could give me some feedback on it , i’d be most appreciative.
Thanks !