Reading your responses to others too, you'll need to find your feet in sales quick before years end. Personally, I have grown and sold a couple of consulting businesses (failed at lots of things too). The key for me was never really which technology we used, it was always about finding a customer demographic I could target and where I had some expertise to help. We crossed nearly all industries but in general I targeted clients with a minimum of $1M/revenue per year up to $50M/year and less than 400 people and privately owned (non-public companies) -- including startups. The key is finding companies who have quick decision processes and who are not as formal as large public corporations generally. We did have a few large Fortune 100 clients as we grew but they were decidedly outside our target clients but we had expertise they would pay well for.

I always recommend people new to sales/marketing read "Crossing the Chasm", it has been continually updated and is essentially how to sell new technology products to the general market. While you aren't selling a product in the traditional sense, you are selling a dynamic product which is you and your skills. So while there will be parts of the book that seem not as relevant it will still help you get a broad understanding of how to market/sell your services. And think of the product as yourself, that's what you are selling when you are a one person dev shop.

I am happy to share my experiences if you have specific questions, I struggled starting my first agency as I didn't have anyone to ask questions to and just had to screw up a ton to figured out what to do. I was super lucky many many times that I didn't tank that business, but was within days of doing so multiple times.