The mail function has the following syntax (see the php manual at www.php.net):
bool mail ( string to, string subject, string message [, string additional_headers [, string additional_parameters]])
For example:
//$header = (your code for the header...);
$message="<html>sender name is <b>$sender_name</b>. blah blah rest of message here</html>";
mail("you@yourisp.com", "hello", $message, $header);
That will send a mail to you@yourisp.com with the subject "hello", headers from $header, and the message from $message. (Note that $message includes a php variable).
Hope that answers your questions.