HOW TO START AN AD NETWORK?

[Jake]

You could find an ad network site clone script, that would at least save a web developer a lot of coding time.
http://www.kaonsoftwares.com/ad-network-script.html

That certainly sounds like something only someone with experience and a reputation in the industry would tackle, trust is one necessary attribute that can take time to earn. Aside from a lot of paid advertising promotion, having mature accounts on the various webmaster, marketing forums would help one reach prospects.
Some ad networks use quite sophisticated ad server software that requires an expensive fast dedicated server.