Form field page with API integration and SagePay Gateway Setup

I need to get a form field page designed to capture customer details (name, address, e-mail address, contact number etc) which once completed is passed via an API to my supplier's backend severs so that the customer account can be created. It also needs to link with the payment process through SagePay which also needs to be setup. I have an idea and concept on how i want the pages to look and feel, can supply my supplier's API details and have already got a SagePage merchant account.

The website itself is currently being designed although I'm currently unable to state in which language (ASP, PHP etc) so I just need help with bringing the above to life.