Small AJAX Script needed

I need a small AJAX script. I have a HTML Form with 3 Fields : 1. A Selectbox, filled by a database query 2. A Text Field 3. A Text Field When i changed the selection of the Selectbox i need the two text fields filled with entries from a database-table, before the form is sended. Example : Database table has the fields : id, name1, name2, city, zip Form fields : organisation (selectbox id and name1 from the table), zip, city When i change the entry of the selectbox to an other organisation the zip and city from the database must be loaded from the record with the selected id and this values must be inserted into the both text fields, before the form is sended. I think this is real simple. You will get access to a special directory of my webserver where you could work on a PHP and (if needed) a JS file to create the needed functions. You don't will have access to the form.