Otherwise, the Arduino does not reset itself. Either way, once you've switched to the serial monitor, press the reset button. This serial communication occurs using RX (pin 0) and TX (pin 1) terminal of Arduino. Any kind of data can send through this serial monitor. Uno, MEGA2560 and Due all have a serial port that connects to the USB device port on the board. This port allows sketches to be loaded to the board using a USB cable. This library receives \n terminated lines that represent a command and arbitrary payload, space-separated. You can tweak it to use your own protocol easily. One that connects through a USB port chip to the USB device port on the board and three extra serial ports that connect to pins on one of the pin headers of the board. A complete introduction to processing is out of scope here, so these two languages share lot of things. Digital In pin and then switch off the LED when the button is pressed and communicate the button state over the Serial interface.I think that it's pretty important to keep low on the amount of informations flowing trough the Serial interface. It seems that most of the how-tos and examples I've found online each loop they print to the serial interface. Doing so we no more need to connect the pull-up resistors to the internal pull-up enabled digital input pins and we can simply connect the button to the pin and ground. In order to implement a simple two-way communication we also have to be able to read from the serial interface. Arduino programming language is modelled after Processing, but we'll see how simple and immediate is that programming language. Code in a sketch can use the same USB / serial port to communicate with the PC by using the Arduino IDE Serial Monitor window, or a Processing application for example. For a complete documentation on Processing you should have a look at the official documentation.
