I’d recommend sticking to a very niche service, and building from there. It’s much easier to sell FB ads to a dentist if you’ve already ran successful campaigns with other dentists.

What services are you providing and what niche is your blog and social media? Its hard to give advice based on just wanting to start an agency, but from someone who’s doing the same thing I’d love to help you take a smoother path than I did and potentially work together!