How Did You Get Started With the Ad Business?

I was running some hobbyist CSS related websites, sharing free code samples. Advertisers would reach out to me and ask to put ads on my site. And so that's how I would make some money on the side for those sites. It got to the point where I was making maybe like $2,000 to $3,000 a month. But it just felt silly to be chasing advertisers down for like 100 bucks here, 100 bucks there.