I'm used to it as I have done a lot of voluntary work involving door knocking. But I still remember when I first started doing it. It IS tough. But once you get a couple of favourable responses it gets a lot easier.
One idea is to pick an area. Put out about 100 leaflets in a chosen street. Next day go back and door knock. Prepare your script! "I left a leaflet yesterday about window cleaning. Did you get it?" Explain that you are looking to increase your rounds. (Don't say just starting up. They might think you won't be any good) Then ask if they would like a quote. If not, tell them to keep the leaflet somewhere in case they change their mind or if their existing window cleaner gives up etc. etc.
One thing,
don't undercut the existing window cleaner, that is a MUGS GAME. You want DECENT PRICED work only. V. Important! Also if you undercut him, and he in turn undercuts you, eventually the job is not worth doing. Who wins? Only the customer.
Reason I say put out only 100 is two-fold. You may find after 20-30 door that EVERYBODY has a window cleaner. If so, try another street. Also you don't want to get fed up. Do your hundred, then take a break. Maybe go out next day.
Keep plugging away. There is plenty of work out there. Let me know how you get on