how to import extension to civicrm
in fact I am going in: administration> system setting> extension


With the above, I think you are able to access CiviCRM on the site. To see the extension in the administration&gt; system setting&gt; extension path, make sure you run the civix command in the extension directory, i.e -


Navigate to Administer -&gt; System Setting -&gt; Directories. Check the value of Extensions Directory input. This is where all your extension should be placed in.
Run the civix command from the terminal in the extension directory.
After the command runs successfully - you should be able to see your extension listed in Administer -&gt; System Setting -&gt;  Extensions path.



  I did a hello world but I do not know how to see the hello appears on the platform


You need to check how we can use extensions to extend the functionalities provided by CiviCRM. Check different hooks that can be used in extension_name.php file to add/modify results on the screen.