For example, why not build a home automation system?
Start with a Raspberry pi and start hooking up peripherals...light switchtes, timers, sensors, etc. You'll stumble across buses like I2C/SPI, you'll learn about networking. You'll figure out what registers are and learn what interrupts are and what they mean.
You'll get lazy rewriting communication code and stumble across messaging frameworks like MQTT to communicate with devices on your network. You'll run out of money using Raspberry Pis for each new device you build, and you'll find cheaper ways of doing things like desiging devices using MSP430 or ESP8266s.
You'll make mistakes, and you will learn. Best of luck on this new adventure!