How to interface with credit card terminals?
The terminals you see in retail outlets are called POS terminals (Point of sale). There is a standard protocol for it (called OPOS) but it is more like a programming project given that the devices (cash register, barcode scanner, rfid reader, card reader, printer) are all standard, and all can be connected via usb or serial port to the terminal PC. The terminal's user interface program needs to be programmed to use the POS protocol. In .net (C# for example) Microsoft has standard OPOS libraries you can use to code your project, but that is not really related to electronics.

You should read up on the OPOS standards for better understanding.