Chris,
Presumably you have a background in advertising and some existing contacts. Working with existing contacts is often the best way to start off, they can then provide referals to others.
Let all your contacts know that you have set up your own business and if they need any advertising help you would be happy to have a chat with them.
Then you need to think about how to get 'new' clients. Are you looking at advertising for one specific market or are you providing your advertising service to all areas. If you are going to focus on one specific area then it makes it easier to focus your marketing on that area.
