To do it the fast and dirty way - simply use the register action itself.

You got all necessary user data already here in place,
you only need to build up a reasonable email body,
then get yourself an instance of the OssnMail class
and use the NotifiyUser function to send that mail to the Owner Email address.

That's it.
And if it's working out the way you want, and if you want it to do the recommended way and leave the core untouched, then copy that changed action into an own component, register it, and revert the core file back to the original.

How do to it myself.
I know how to do a send of mail with php but when i declare my own variable (with the 'simple' mail() function) but i didn't found where and how the variables are declared.

The variables are these lines ? (found in actions/user/register.php)

$add->username = $user['username'];
$add->firstname = $user['firstname'];
$add->lastname = $user['lastname'];
$add->email = $user['email'];
$add->password = $user['password'];

if i want to make my own mail() function, how must i declare it ?

<?php 

(mail([email protected])){
echo 'A new user subscribed on your website.<br />';
echo 'His pseudo is ' . $user['username'] . '<br /><br />';
echo 'His email is ' . $user['email'] . '<br /><br />';
} else {
echo "Error";
}
?>

> I put this email address in the notification field in admin panel

Hm, it sounds like you got it wrong. The Site Notification Email is not a target, it's source address.
When I register to your community, I will receive a mail from the address you entered there. Not you.

What you want, is an additional email with the credentials of the new user being sent to the Owner Email address. But that's currently not implemented I think.