Advertising is a big step. As with any trade, advertising has its' prooven methods, as well as its' failures.
How big an ad? What sort of type? Pictures? Art? These are basic questions- and, believe it or not, there are answers! Answers supported by decades of tracking expereince.

Just don't count on the sales rep for guidance. While they may know all the rate schedules and promotions, as well as be able to fill your ear with tales of how so-and-so does it "this" way...well, in my experienct these folks are there simply to get the sale, and have NO actual training in, or experience with, advertising.
Indeed, many are the tales of dollars spent- without result. Or, even, bad advice that actually reduced sales.

I reccomend that anyone wishing to advertise get the little book -almost a pamphlet- "My Life in Advertising / Scientific Advertising" by Claude Hopkins. This book is one of those rare things that holds true even decades after it was first printed. Better yet, it's available for about $10, from any book source.