The architecture of Arduino UNO (Read about ATmega328P also)
The architecture of Arduino programming ( understand the stepup and loop functions)
Read upon GPIO functions and Pins ( General purpose Input Ouput)
Now you compile and run the blink sample code
Now take one more step into peripherals - UART (serial communication); What happens when you use Serial.print() function
Learn how to change the blink intensity by varying voltage (PWM - Pulse width modulation)
How to communicate with external sensors ? Using SPI, I2C or UART
How to control external transducers
The best way to learn is by doing some project. Take a real world problem and try to make an Arduino based solution for it. This may require multiple sensors and transducers. That will make you learn about different communication protocols. Then you will learn how to use them with the Arduino.
