Before you can add domains/websites to your account using WHM you need to create some packages. These will define the amount of resources you wish to give an account.
In you web hosting manager, in the menu on the left, click on 'Add packages.'
In the form that follows fill in the resources you would like to give the package, for the average website 100 MB of Space and 1000 MB of Transfer will more than enough.
Note: It is not possible to create packages which allow unlimited space or transfer, as such a thing does not exist
When you are finished click ‘create’ and you are done.
Now, when you are adding accounts you can choose this package.