After creating a setup() function, which initializes and sets the initial values, the loop() function does precisely what its name suggests, and loops consecutively, allowing your program to change and respond. Use it to actively control the Arduino board.
const int buttonPin = 3;
// setup initializes serial and the button pin
// loop checks the button pin each time,
// and will send serial if it is pressed
if (digitalRead(buttonPin) == HIGH)