As you know Arduino Mega has 4 serial ports. I need one extra serial port besides the one which is connected to computer via usb cable. So I want to use one of the other 3 serial ports. But they are just pins. Is it possible to use any other serial port with usb to connect to PC and if so how can we do that? How can I connect those two RX TX pin to a usb for connecting to computer?
You need an FTDI cable, for example, this one from Sparkfun on each additional serial port you want to use, connected to whatever serial device you want it to talk to.
If the target serial device is the same computer you're developing code on, you'd connect the cable to an additional serial port on that computer. For OS-X, you're done. For Windows, you probably need an FTDI driver for the additional port.
The example cable has a 6-pin female connector. You'd to wire its Rx & Tx pins to your Mega's Rx1 (for instance) and Tx1 pins, and its ground to one of the Mega's ground pins.