I think you’re in for a longer journey, but if you are motivated and have a bit of patience, I am sure you will get to your goal. Its not a “first timer” kind of excecise, but its also not rocket science, so I am sure you can make it if you want to.
I think Valentine’s suggestions are very good. You need to get familiar with the basics of I2C, Arduino and working with the Arduino IDE, and following these kinds of tutorials is a good way to do it.
As you do so, you can keep your ultimate goal in mind, and see if you can adapt the examples a little bit, in small ways, to bring yourself closer to the goal.
When working with I2C, a logic analyser is a very useful tool. If you don’t already have one, then something like this: https://www.amazon.com/KeeYees-Analyzer-Device-Channel-Arduino/dp/B07K6HXDH1/r is sufficient for I2C, together with the free software “PulseView”, that’s a cheap I2C debugging solution.
You will also need to decide on cables, here I would recommend the Stemma/QT aka Qwiic standard - it is I2C over 4 wire cables with JST-SH-1.0 connectors. If you follow this convention you will find cheap pre-made cables - single plug, double plug (buy the ‘reverse side’ kind), and also helpful ready-made boards like this or this.
Also Sparkfun and Adafruit make various Arduino-compatible MCU boards like the “Thing Plus” or “Feather” boards, many of which come with Qwiic I2C connectors already on-board.
Anything you can do to simplify your life regarding cables, soldering, pin-choices etc will eliminate sources of error, and make things much easier for you.
So you could imagine ordering a few Thing-Plus boards with on-board I2C connector, the Qwiic Multi-Port adapter, and then connecting everything up with some ready-made cables. Then you could immediately start testing I2C communications.
When testing, you might want to consider ordering a USB hub and some extra USB cables. Then you can connect all the MCU boards to the same computer at the same time, and watch the Serial output for all the MCUs at the same time. This will make it much easier to debug, since each MCU can print the data it sends and receives to its own serial port, and you can compare the debug messages to see what is going on.
These are just some top level ideas… give it a go, and let us know how you’re doing with it!