Here is how we do it in one of our apps:
$to = 'admin_email_address@mysite.com' ;
$from = "registration@mysite.com";
$subject = "New registration";
$message = "There has been a new registration on the site";
$headers = "From: Site Registration <".$from.">\r\n" .
"Reply-To:".$from."\r\n" .
"Return-Path: <".$from.">\r\n" .
"X-Mailer: MySite Registration Module";
mail($to, $subject, $message, $headers);
As far as where to place it? I'd look in my code where the new record is being inserted into the database, or when the newly registered user is notified of a successful registration and place it there.