const int buttonPin = 0;     // the number of the pushbutton pin
const int ledPin =  1;      // the number of the LED pin

// variables will change
int ledState = 0;         // variable for ON/OFF led

void setup() {
  pinMode(ledPin, OUTPUT); // initialize the LED pin as an output
  pinMode(buttonPin, INPUT); // initialize the pushbutton pin as an input
  digitalWrite(ledPin, LOW); // setting up led OFF for beginning
}

void loop() {
  if (digitalRead(buttonPin) == LOW){ // checking if button is pressed
     if (ledState == 0){ //if led is OFF
      ledState = 1;
      digitalWrite(ledPin, HIGH); // setting led ON
     }
     else { // if led is ON
      ledState = 0;
      digitalWrite(ledPin, LOW); // setting led OFF
     }
     delay(500);
  }
}