First of all, read basic information about Hosting Plans here. Next, read Hosted Organization and Exchange quotas explanation.
Go to Account > Hosting Plans
Click Create Hosting Plan button
Edit Hosting Plan page will appear. Give this hosting plan a name and select previously created Virtual Server as a Target Server.
Wait couple of seconds and you will see that System, Service Levels, Hosted Organizations and Hosted Exchange quotas are available for selection now
We need only 3 of them: System, Hosted Organizations and Hosted Exchange.
Setup limits according best practices and your wishes. Click Save button at the bottom of the page.
Done, next step is Customer creation, then Hosting space creation.