If you are thinking of setting up a web marketing agency then you need to think carefully about how you are going to run it. It may be that you are going to only use freelance designers, marketing experts and programmers or you may decide to employ a number of people to work within the business. Ideally, you will need a central hub that everyone can work from, as even if you are only using sub-contractors, ideally you want them close at hand so you can discuss projects and keep up to date with what they are doing.
When looking for clients, you need to decide how you are going to market your web design and marketing agency. The first thing you need to do is have a great website and a good presences on the social media sites. You may also decide to attend some networking events to give out business cards and drum up some business.
Be sure to check what your competitors are able to offer and also what they charge as this will allow you to check that you are competitive.
Results are often a huge selling point when it comes to marketing, so you may need to build up a good portfolio before you can tout for larger contracts etc.