You can quote by square foot but always insist on doing an walk-through of the place that’s going to be cleaned before you give a final bid. You can give an estimate and let them know you’ll need a walkthrough just to make sure before any contracts are made. You might find out the place is not what you expected and may take alot more time to clean than you figured… that’s the name of the game with cleaning: the faster you can clean it, the better so you can move on to other jobs on the same night etc. As for getting the accounts, find out what types of places you want to clean. Offices? Offices in industrial areas/airports/cargo buildings, factories, etc. Get some business cards and things like that and either cold call (sales call out of the blue) or go in person to the places.

First decide how much you will charge per square foot or per bathroom stall or whatever you plan to do you price structure. Then cold call to building managers asking to be allowed to bid the work. Make a form to record the room sizes and what work they want done then go measure the spaces and ask what they want done.
Figure what time it will take you can what the cleaning peoples time will cost you including any benefits your provide.