I was a nanny (10 plus years) before I opened my day home. I would not give a decent chunk to an agency. And to teach me what? I know how to look after children and dealing with parents is not diffucult. You just have to force your contract if there is any problem.

Just be confident( not saying you are not) and start. There is a lot of information on this web site and if you need help for particular situation ask here. We all are lovely people who are willing to help. The best part is it is free!
Think of what you can do with the money you are willing to pay to an agency. Give you a hint. Shooooopppinggg