How do I promote my web design agency?

That's a great question, because over the last couple of years we went from a place where you really needed to hire somebody to be able to build a website and now every software tries to say how you don't need to hire a web designer to build your website. If you want to sell your services, you need to show what you offer that's different and more professional. Paid advertising is certainly one way to do it, but you need to have a game plan going into that. If you can, try creating videos and posting them to YouTube to help your chances of getting found and making your work being seen.