Adding additional content to Delivery Address

[brookbab]

I was wondering if anyone knew how to add additional content to the Delivery Address. For example at the moment the site asks for the delivery address and postcode etc. but i need to make it ask for the telephone number and for it to obligitory.

 

Which files do i need to amend besides...

 

admin/classes/order.php

classes/order.php

checkout_shipping_address.php

 

THANX

[brookbab]

BUMP