Well your first step would be to research the topic of
Authentication and Authorization. There are lots of ways to authenticate a user, but if you use Forms Authentication with
Role Management it will help you determine if the user is an "admin".
From there you can determine if the logged in user is an administrator and give permissions to the user to edit content.
This is a large topic and will take a while to research.
Make sure you pick the authentication type that is best for your specific application's needs.