Press a button to start

wait_push_button_to_start.pde

#define PIN_ONBOARD_LED 13  // DIGITAL
#define PIN_START_BUTTON 12 // DIGITAL

void setup() {                
  // Pin 13 has an LED connected on most Arduino boards.
  pinMode(PIN_ONBOARD_LED, OUTPUT);
  pinMode(PIN_START_BUTTON, INPUT);

  wait_start();

  // Here is safe to use the serial
  Serial.begin(9600);
}

void wait_start() {
  digitalWrite(PIN_START_BUTTON, HIGH); // turn on pullup resistors
  int state = HIGH;
  while(digitalRead(PIN_START_BUTTON) == HIGH) {
    digitalWrite(PIN_ONBOARD_LED, state); // turn the onboard led ON/OFF
    state = !state;
    delay(100);
  }
  digitalWrite(PIN_ONBOARD_LED, HIGH); // turn the onboard led ON
}

void loop() {
  Serial.println("OK");
  delay(1000);
}