Payment Gateway Integration - 3rd party cart

I'm utilizing a 3rd party shopping cart as my billing software. It does not ship with the payment gateway module I need to process clients' credit cards. I have documentation from both the shopping cart system, and the payment gateway on how to do this. However, I simply don't know how to put the code together as I have no php experience. Essentially, it sounds like this may be just a bit of copy and pasting. And I can research additional information from both the cart developers and the gateway if needed. This is not a system that sends users to another site for payment. It uses the gateway API to process credit cards in the background, while keeping clients on my website.