I assume you are using Windows.
Plug in your Arduino to PC using your usb cable. Open Device Manager. Look for ports(COM & LPT ). There you will find your Arduino identified. Mine is COM3. Look at "Port Settings" mine are 9600,8,none,1,none.
Now use this code:
As long as your port is set to 9600 this will work - if not modify the C# code to your baud rate at this line:
currentPort = new SerialPort(port, 9600);