Seems like there are two questions here... 1. how to start a social media advertising agency, and 2. turning it into a powerful marketing and advertising agency.
To answer #1 I would say just start. Do you have experience in social media and advertising? If you do then start reaching out to local manufactures and offering them your services (perhaps sharing an ebook or inviting them to a webinar teaching about social media marketing). If you don't have experience I would start to learn more. Take a course or find a job at an agency to gain experience. Your potential customers are going to want to work with someone who knows the industry well and can provide them with good results.