A photoresistor is a sensor that reads light levels and converts those levels to a number value.
You can take that value and have it control an LED using an Arduino.
Start by following this tutorial to wire up a photoresistor, Arduino and one LED.
The tutorial has a wiring diagram and sample program.
Graded assignment: Once you have this program working do one of the following:
1. add two more LEDs and have them turn on or off based on the available light sensed by the photoresistor OR
2. wire a piezo buzzer and program the buzzer to change tone based on the amount of light detected.
For either option, you will need to use Serial Monitor to see what values are being delivered by the photoresistor.