I need to add a internal pull up resistor to my code but I'm not sure how

I need to add an internal pull resistor like I said to pin 12 but Im such a newbie at code that I have no Idea where or how?

Here’s my code:

int LEDArray[] = {5, 6, 9, 10};           // LED array for PWM 5,6,9 and 10
int numOfLEDs = 4;                       // LEDs of index 0-3
int pirState = HIGH;
int pirVal = 0;
int pirPin = 12;


void setup()  {
 
  Serial.begin(9600);
  pinMode(pirPin, INPUT);
 
 
  int i;
  for(i = 0; i < numOfLEDs; i++) {
    pinMode(LEDArray[i], OUTPUT);
  }
}

void loop()  {
  pirVal = digitalRead(pirPin);
  int i;
  //if motion is captured, light the LEDs in sequence
  if (pirVal == HIGH) {
  for (i= 0; i < numOfLEDs; i++) {
    analogWrite(LEDArray[i], 300);
    delay (25000);
  }
  delay (10000);
   for (i= 0; i < numOfLEDs; i++) {
    analogWrite(LEDArray[i], 0);
    delay (10000);
  }
  if (pirState == LOW) {
      pirState = HIGH;
    }
  }
  //else, set all LED values to zero
  else {
    analogWrite(LEDArray[0], 0);
    analogWrite(LEDArray[1], 0);
    analogWrite(LEDArray[2], 0);
    analogWrite(LEDArray[3], 0);
    if (pirState == HIGH) {
      pirState = LOW;
  }
}
}

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