Stand back from the door after knocking. When the householder opens the door I smile and say 'Hello, nice to meet you. My name is John and the reason I am calling is that I am offering a window cleaning service locally. Would this be something you may be interested in?'
Keep it simple. Be presentable, polite, professional, but don't be pushy. There is no need to apologize for calling, but you can simply thank them for their time if they are not interested.
Best wishes,
John