Each Wireling has example code that you can test with your Arduino IDE. For example, the 0.42" OLED Wireling has two Download files with:
* Arduino Sample Code
* Arduino library
https://tinycircuits.com/collections/wirelings/products/0-42-oled-screen-wirelingOnce you've gotten comfortable with the Moisture Sensor and Display examples, you'll run into another question, which is how to display the data from your moisture sensor. As you've noticed, Wirelings use I2C. Each Wireling has its own I2C address. To talk to a Wireling you typically set the I2C port:
Wireling.selectPort(portForMoistureSensor);
// read data
Wireling.selectPort(portForDisplay);
// write data to display
That's a simple example.
As your project grows, you'll start adding more Wirelings. Yay! Wirelings are fun!
I've got three Wirelings hooked up to my TinyZero via the Wireling I2C Adapter Shield.
I'd add a fourth, but the USB micro plug gets in the way (sadness).
I look forward to seeing your project. I'm guessing you're doing a gardening project and I'd love to see it!