We all have to start somewhere. We in effect have to sell ourselves to potential customers. You could just say 'Hello, I am starting a window cleaning round in this area, and wonder if you would be interested...'
You don't have to say how long you have been a window cleaner.
When I started to build my business I had just 3 months experience (I was taught by, and worked with a friend). Nobody asked how much experience I had. If they had I would have been truthful to them.
My tip for you is to be honest. Trust takes a lifetime to build, and an instant to destroy.
John.