You need at least two user levels. One for admin and one for users.
When people first register (so on the registration form) you need to add a hidden field and give it a value equivalent to the users level. Then all registered users will have that entered into their details.
To create admins you can either set up something in an admin section of the site where you can change their user level to admin or (and this is the easiest option) just go into phpMyAdmin and change it in there. It all depends whether this is for you or a client.