You will need to use an external library such as PHPMailer. This is pretty simple, because you just include a php file then use their functions for sending an email instead of the default mail function.
Here is an example of what you might use.
require_once("/your/path/phpmailer/class.phpmailer.php");
$mail = new PHPMailer();
$mail->From = "your@address.com";
$mail->FromName = "Your Name";
$mail->Host = "localhost";
$mail->Mailer = "mail";
$mail->Subject = "Here is your HTML email!";
$mail->Sender = "Your Company"
$html_body = null;
$html_body = urldecode('
Sweet!
');
$mail->IsHTML(true);
$mail->Body = $html_body;
$mail->AddAddress('recipient@address.com');
$mailed = @$mail->Send();