void setup() {
  pinMode(13, OUTPUT);
  pinMode(2, INPUT_PULLUP);  // No external resistor!
  Serial.begin(9600);
}

void loop() {
  Serial.println(digitalRead(2));
  if (digitalRead(2) == LOW) {  // Button pressed
    digitalWrite(13, HIGH);
  } else {
    digitalWrite(13, LOW);
  }
}