How to track what information the program sends to the server?

Question

Server equipment

Hello.
There is a server that performs a certain function, I won’t tell you a secret which one. Using a local network, several stationary stations are attached to it. As well as on the server so at stations of Windows HP OS. Stationaries are cash desks.
The cashier at the checkout creates through a specialized program, let's call it 'SON', a certain cell with information and sends it to the server, let's call the server "DAD". DAD sees this command and proceeds as it is prescribed, it is no longer important to us further.
So here is my question.
How can I see what SON - DAD sends without the visual participation of these programs. That is, at the machine-software level.
What articles should I read for this, what language should I start learning so that the computer can understand me.
Let me explain why I need it. SON and DAD are technologically complex programs, and in order not to delve into someone else's code, I decided that it would be easier to simply deceive DAD to pass away as a SON and send the information I need to the server from the same cash register.

Answer 1

The computer does not think at all, it only executes the program. So, if you want to understand what is currently being executed - a disassembler and a debugger in hand.
If you just want to pretend to be a client and the traffic is not encrypted - wireshark / tcpdump and reverse packet analysis.

Answer 2

wireshark