Just pull the info from the db and add it to the variable:
$query = "select XXX from TABLE where XXX=XXX";
$result = mysql_query($query, $db_connection);
$message = "xxxxx";
$message .= "\n\n";
while($row = mysql_fetch_row($result))
{
$message .= $row[0] . "\n";
}
mail($to, $subject, $message, $header);
That will take each row from the database and put it on a new line in the $message variable.