oh ok, from your post i couldn't tell if you knew any php already.
in my experience, you will usually need to have an account setup with any company that provides an API whether it be UPS, Google, or of course a company who deals with money. most of the time they do have test areas you can work with while you debug your apps.
checkout the CURL library and if your server supports it, then use it. look at the examples under curl_setopt() and curl_exec()
if you wanted to play around with it before you get a worldpay account setup, just create a php page that populates some xml (or whatever) with post data.
then create your curl script to send post data and retrieve the text. this is the basic mechanism when dealing with e-commerce like this.
good luck with your project.