Don't start a marketing agency, everybody and their grandmother start a marketing agency nowadays, and they fail because it is hard to differentiate yourself and most often people don't know their stuff on the implementation side. My advice is to stick with what you know, hire slowly and fire fast. Additionally niche down to a specific service in a specific industry/vertical. For example "writing blog content for heart surgeons". Everything will make you money as long as you deliver results and you have clients on retainer. Think about your service and your offer - how can you make an irresistible offer that prospects can't refuse. This is how you differentiate yourself and make money.