You may try all sorts of ways; internet, social groups, business groups, networking, advertising (all types however local will be best and less expensive), etc.
However the best way to get started is to knock on doors Hickson. This allows personal contact, experience, observation of the building/facility, etc. Dress for the part, be nice/friendly, ask the proper questions, set appointments, etc.
No one comes breaking down your door begging you to please clean their buildings. You need to work hard for it especially in the beginning and do you due diligence.