If you’re worried that someone else might know more than you do, stop. I’m sure someone will, but that someone is not your client. There are many different levels of knowledge on any given subject. All you need to know is a little more than your client.
When I started fixing computers I learned everything I knew from a book. I just had a basic understanding of how a computer worked and had some experience with windows. I made a lot of mistakes and did not charge a lot of clients in the beginning, after all, if I had to learn at the client’s expense I couldn’t charge him. But I knew what to do when that same problem showed up for another client.
That learning curve got me more experience than anyone else I knew that had formal training. I was able to know what’s wrong with a computer just by working with it for a minute or two. And I was always right. Not because I’m so smart, but because I resolved an issue just like that.
The point of this story is that you can start working with minimal knowledge and still be able to help some people. The more experience you earn the more people you can help and your client base will grow accordingly. So now that you one less excuse what’s holding you back?