You can use the Arduino environment's built-in serial monitor to communicate with an Arduino board. Click the serial monitor button in the toolbar and select the same baud rate used in the call to begin().
The Arduino Mega has three additional serial ports ... (Don't connect these pins directly to an RS232 serial port; they operate at +/- 12V and can damage your Arduino board.)
Uno only has one port on pins 0,1 but presumably the same proviso applies. An RS232 to TTL level shifter cct is required.