0. Do not confuse authorization and authentication.
Just the fact that the user is authenticated (you know who they are) doesn't mean you should do whatever they ask you to do.
1. Consider resource-based authorization.
In many cases it makes sense to model authorization as a matter of certain users having special access to specific resources. E.g., we could designate some users as being "owners" or a particular resource and give them special access to this resource based on that fact.
2. Consider role-based authorization.
In other cases it makes more sense to assign users to "roles" and link permissions with those roles.
It's usually hard to get away from resource-based authorization entirely, so in practice role-based authorization often ends up being used in combination with resource-based authorization.