I2C nodes with arduino

Hello everyone,
I am a last year student in ICT and i need to make a big project together with a friend of me so we wanted to try to make a smarthome without a premade OS system(It's scaled and only with 5V electronics). So i downloaded Raspbian on the raspberry pi and now i am using Node-Red and Blynk (for the webserver and app) but my teacher wanted us to use I2C instead of just using every gpio pin.
Now i want to use a arduino pro mini with my raspberry pi and node-red but i don't know how i can call different programs on the arduino with switch cases. I use the i2c node but don't understand how to node works i got it to work to put all the pins of the arduino on but i can't make different cases in the arduino to control different leds.

Can someone help please (I am dutch so my English isn't the best, sorry for that)

Have a look at Peter Scargills blog - he makes extensive use of Arduino Minis/Nanos as slaves to ESP8266 devices and makes his source code readily available - this should give youa good starting point