Mi first step was to make things work, one thing at a time.

I started with the DHT22. Very easy in micropython as the library is already included.

Then I stepped to the OLED 0,96" screen. Try to show some simple messages using the library from https://github.com/micropython/micropython/tree/master/drivers/display

Then I stepped to MQTT. Once you understand the way it works, it won't be problematic but when I put everything together had some memory issues. I had to keep the OLED out for a while until I figured out how to maintain my memory usage low.

The best tutorial I've found to understand MQTT is this one: https://www.linux.com/news/mqtt-building-open-internet-things

The last thing is OpenHab. I'm into this at the moment...