setup()

The setup() function is called when a sketch starts. Use it to initialize variables, pin modes, start using libraries, etc. The setup function will only run once, after each powerup or reset of the Arduino board.

int buttonPin = 3;
 
void setup()
{
  Serial.begin(9600);
  pinMode(buttonPin, INPUT);
}
 
void loop()
{
  // ...
}
 

Source: arduino.cc