Use case for admin and simple user

hi im new to uml and i want to do simple use case diagram
sample: online user only show and look site content like announcement,news…
administiritiors logon site and has some authorization for doing update,delete,insert news,… and create a new admin and give authorization.
how can we do a use case diagram for this site.
for example we only create one actor and include login and authorization check.
we can create admin and simple user aktor…
i confused about it