How are advertisers paid for their services?

Im looking to start a small advertising company and I am wondering how the company will be paid by their accounts. Do you receive a percent of sales if it can be traced back to your advertising as the reason for a new sales account? Do you get paid a lump sum up front for ad expenses and keep what is left after advertising? How do you pitch an ad and how does copywriting a slogan work? Does the creator and company own the right to such slogans or copywrited pictures?