I am a computer software student. Using assembly language programing I can throw output signals on serial or parallel ports to control devices using computer.

But what I want to know is that the output digital signal will give 5V DC and it will not work to run an AC 220V motor.

How can I make circuits to match that voltage? what should I read to do this.

 

If you are thinking of using the RS-232 COM ports or Centronics-compatible LPTx parallel printer ports, you will either need to use some flavor of Unix, Windows 98 or earlier, or some version of DOS; as Windows 2000 and up does not "approve of" programs directly accessing hardware. Besides that, RS-232 and parallel ports are considered "legacy", and are no longer included on many new computers.

If you want to be able to run your program on later Windows platforms, you will need to use calls to the OS to handle your I/O requests. You would be much better off to write programs that will use USB, as the data throughput is far greater than you could get via either RS-232 or the LPT port.

And no, you cannot directly control mains power using signals from a PC.

Here is an example of a USB I/O interface that controls up to 8 relays; the relays can switch loads up to 5A @ 250VAC:
http://www.sparkfun.com/products/9526
At $145/ea, they are not cheap. However, you could spend a lot of time trying to make something less expensive, and it most likely would not be as safe.