How to add a button to report (flag) some content item to an administrator for that content? I would like to print near a given type of content a button "report" that, when pressed, send a notification to the admin related to that content.

In this specific case, I have a textual field with a link and I would like to allow users to be able to report (signal) some content item to the administrator if some link is not working anymore. So that the administrator can fix it.
You should investigate the flag module https://www.drupal.org/project/flag and Rules https://www.drupal.org/project/rules (remembering to use version 2.7 of the latter if using drupal commerce) that should allow you to create a "report" flag and send a notification to an admin.