There are several other threads about this topic in the forums, worth doing some research.
Anyhow, one simple approach is to add a Yes/No field to the user datatype e.g. isAdmin = yes/no
On the privacy tab, then create a Role Admin, and set the rule current user isAdmin is yes, then set the relevant options. and for the Everyone Else role set them as appropriate.
On the client side (the pages), you can then also check (conditionals on elements) the user isAdmin field and show/hide elements as appropriate. You can also manipulate elements, when a user logs in/out via the workflows and carry out checks against the isAdmin field.
You can also restrict access to pages, on page load workflow, by checking the field.