#define LED 3
#define BTN 2

void setup() {
  pinMode(LED, OUTPUT);
  pinMode(BTN, INPUT_PULLUP); // this keep HIGH by default
}

void loop() {
  int btnVal = digitalRead(BTN);

  if (btnVal == LOW) {
    digitalWrite(LED, HIGH);
  } else {
    digitalWrite(LED, LOW);
  }
}