Sites are the top level of the organization within an account.  Many details of your cleaning programs are defined at the site level. 

Log in to your account and go to "Account Setup"  here you will be directed to "Account Info / Sites".  

Adding a new site is simply done by clicking the plus icon next to Sites.

Give your site a fitting name, here we'll use "Green Quarter".
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If you don't know how much cleanable area you have that is ok,  this value is simply used as an estimate to calculate things such as Full-time employee hours.