I think you are in some trouble if you took this on without XC5 experience. Its going to be a big uphill journey trying to understand.
I'll do my best to help get you up to speed. First you need to create a module. Do you have a custom module for the client created yet? If not follow this tutorial:
http://xcartguru.com/tutorials/x-cart-5-tutorials/building-a-custom-skin-in-x-cart-5/
It teaches you how to build the bones of a custom skin which is also a module. Get that done, then you will have a place to start from.