After creating my extension, what is the root for my new tab?
It appears you are confused about the difference between a CiviCRM Extension and a WordPress Plugin.

I would recommend you to write a native CiviCRM extension and enable it from the civicrm/administer page (you might need to set up the extension directory path first).

If you generate an extension with civix, it will take care of reading the xml to handle the url of the page you created and all the stuff you'd need to support your custom tab