int LED = HIGH;
int inc = 0;

void setup() {
  pinMode(13, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  digitalWrite(13, LED);
  if (Serial.available() > 0) { //Czekamy na serial port
    inc = Serial.read();
    if(inc == 49) { //49 to 1 na klawiaturze w ASCII
      LED = HIGH;
    }
    if(inc == 50) { //50 to 2 na klawiaturze w ASCII
    LED = LOW;
    }
  }
}