This is a good tutorial on Advanced AJAX with MySQL
http://www.tizag.com/ajaxTutorial/ajax-mysql-database.php

The request URL would have _GET varaibles in, just create the file action.php with the contents

<?php
print_r($_GET);
?>

and view the output in Firebug (XHTML requests section)

Sending data:
The easiest way to send data to your server from the form is using _GET variables
You can take the variables from your form and add them to your string like this

function action()...