int lastButtonState = HIGH;
bool ledState = LOW;

void setup() {
  pinMode(7, INPUT_PULLUP); // Taster
  pinMode(12, OUTPUT);      // LED
}

void loop() {
  int p = digitalRead(7);

  if ((p == LOW) && (lastButtonState == HIGH)) {
    ledState = !ledState; // toggle
    digitalWrite(12, ledState);
    delay(200); // delay
  }

  lastButtonState = p; //memorize former value
}