const byte led = 9;
const byte button = 3;

boolean lastButton = LOW;
boolean currentButton = LOW;
boolean ledOn = false;

void setup() {
  pinMode(led, OUTPUT);
  pinMode(button, INPUT);
}

boolean debounce(boolean last)
{
  boolean current = digitalRead(button);
  if(lastButton != current)
  {
    delay(5);
    current = digitalRead(button);
    return current;
  }
}

void loop()
{
  currentButton = debounce(lastButton);
  if(lastButton == LOW && currentButton == HIGH)
  {
    ledOn = !ledOn;
  }
  lastButton = currentButton;
  digitalWrite(led, ledOn);
}