byte pinButton = 3;
byte pinLED = 4;

void setup() {
  // put your setup code here, to run once:
    pinMode(pinButton, INPUT_PULLUP);
    pinMode(pinLED, OUTPUT);
    digitalWrite(pinLED, LOW);

}

void loop() {

  if(digitalRead(pinButton) == LOW)
  {
    digitalWrite(pinLED, HIGH);
  }

}