I would like to start a cleaning service for office buildings, banks, est... But don't know where to even start.

Start with a market study, find your niche and exploit it.
ASK around and see what others are doing.

Thank you for the help now after that, is there a website that I can look at to get contracts to do the work?

Simply walking around your local business area, reading telephone books and newspapers.
After all YOU know your target audience better than anyone.

For a contract to use, Google "cleaning services contract sample" (without quote marks). Here are a few I found (to be adapted for your own use) --

Cleaning Contract

Simple Contract

How to Write A Contract