void setup() {
  pinMode(2, INPUT); // pin 2 connect into button and detect change from button
  pinMode(4, OUTPUT); // pin 4 connect into relay and change Relay situation
}

void loop() {
  int pb = digitalRead(2); // Read/detect change from button
  if (pb==LOW){ 
    /* 
      pb==LOW (Pull down) GND connect to push button and VCC/5V connect into resistor
      pb==HIGH (Pull UP) VCC/5V connect to push button and GND connect into resistor
    */
    digitalWrite(4,HIGH); // Set Relay On
  }
  else{
    digitalWrite(4,LOW); // Set Relay Off
  }
}