It takes time and a LOT of effort to build a round. Start by looking as professional as possible,.. some kind of logo'd top or uniform is always good. Have LOADS of business cards and give them out at every opportunity, as well as flyers.
Take a look at where you live and see where there is a need for a window cleaner. Most shops have one already, as do most offices. Its often easier to pick up domestic work in the short term.
Plan for the long term though, get your vehicle signwritten, get a website setup etc and the commercial work might start looking for you,....
And NEVER EVER nick another cleaners work, especially by undercutting. Be friendly with local cleaners, ad respect the hard work they've already put into building up their round, ask where their "patches" are,... and maybe they'll send work your way too when they're under pressure!