Nicky - It's hard to answer your question without some specifics....
Can you be more specific about the "work" you require from the agency? Do mean create the ad? Or produce it? Or place it in the media? What kind of ad do you want to make - digital, newspaper, outdoor, direct marketing, tv/radio etc....?
Do you have a minimum budget in mind in terms of what you can afford spend up front with the agency? And then what kind of revenue would you expect in the first year?
Also, is there any chance you could desribe the product or service? If it's all hush hush, can you at least categorise it?
Bob