Create the following circuit:

Enter the following code (note that you may need to adjust the threshold value, depending on your room's ambient lighting): 

// Pins
const int light_pin = A0;
const int led_pin = 6;

// Set darkness threshold
const int threshold = 900;

void setup() {
  pinMode(led_pin, OUTPUT);
}

void loop() {
  
  // Read light value
  int val = analogRead(light_pin);
  
  // Turn LED on if light value is below threshold
  if ( val < threshold ) {
    digitalWrite(led_pin, HIGH);
  } else {
    digitalWrite(led_pin, LOW);
  }
}

Simulator: https://tinkercad.com/things/cHGrr0pl7Zp