In the user table, you should have a default value for the user level column. I would recommend the lowest user level, 1. This way all users are set to the lowest level automatically when they register.
It is up to the site administrator to manually elevate a user to another level. The site administrator should decide which users are power users or admins.
The initial home page they are directed to will not change based on the user level, you can add additional content such as links to administrative parts of the site that will only show to elevated users.