What Type of Attack Should I Use?

Hello, So I'm planning to start ethical hacking by myself. For my first project this is what I want to do:

So I wanna do something to an Ubuntu server of mine. It accepts some json POST request (by one of the applications I made) so what I want to do is intercept that json object and modify it :) but I don't know where to start, I know programming, C++ and python for this project. What type of attack should I use?