When I started out in photography, I didn't even know that ad agencies were a thing. To be honest, I didn't know a lot at all. However, over the years I have picked up a few tricks that will hopefully help you.
Ad agencies vary in size, from one person working from home to huge umbrella agencies that own several agencies around the world. For each of these a slightly different approach is needed. Knowing who you are talking to at the agency can be crucial too.
In the commercial world of photography, the photographer is usually employed by the creative team, who are in turn employed by the brand's marketing team. Sometimes brands do this work in-house, but for the bigger jobs, or smaller brands who can't afford an in-house team, this generally is the approach. Before getting to this point, agents take their photographers' books in to show the agency and to try to set up meetings with key art buyers and directors on behalf of the photographer.
In this video, I talk about who those various people are in the agency, which ones you should contact, and when. From art buyers, to creative directors, and account managers. Getting the right contact at the right level of agency is key. Although the bigger places are used to meeting with agents, they do also meet with photographers directly and smaller agencies will always meet with photographers, so building a good relationship is key.
Also, the cat was fine at the end of the video, don't panic, she sleepwalks a lot.