int led = 13;
int btn = 12;
void setup() {
  // put your setup code here, to run once:
pinMode(led, OUTPUT);
pinMode(btn, INPUT_PULLUP);
Serial.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly:
int buttonState = digitalRead(btn);
if(buttonState == LOW) {
  digitalWrite(led, HIGH);
} else {
  digitalWrite(led, LOW);
}
Serial.println(buttonState);
}