How does an advertising agency get paid?

I worked at a radio station selling advertising. When I would ask my supervisor and boss how a marketing / advertising agency works, the answer I would get is…it's not worth your time or hard work. I have since quit my job due to, well, my boss was a tyrant, but that's irrelevant. I have run into a couple of my old clients that have asked me to handle their advertising, but I have no idea how to charge them or how I get paid by the advertising avenues. Can someone please tell me, how does an advertising agency get paid? Please be specific…thank you!