Voltage Dividers
https://learn.sparkfun.com/tutorials/voltage-dividers
Resistor Color Code Calculator and Chart
http://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-resistor-color-code-4-band
Diode and LED Polarity
https://learn.sparkfun.com/tutorials/polarity/diode-and-led-polarity
Basics > Analog Read Serial (basic potentiometer circuit)
https://www.arduino.cc/en/Tutorial/AnalogReadSerial
Basics > Blink (basic LED circuit)
https://www.arduino.cc/en/Tutorial/Blink
analogWrite() (fun with PWM)
https://www.arduino.cc/en/Reference/AnalogWrite
Basics > Fade (LED with PWM demo)
https://www.arduino.cc/en/Tutorial/Fade
Seeing the Light: Using Photoresistors (LDRs) with an Arduino
https://blog.udemy.com/arduino-ldr/