Ranjit, its not very hard to do. The easiest way is to use timer 2. You need to remember that you have two settings to worry about, the basic reload count which will determine the pulse rate of your square waves and the compare count which will switch the output when the count is reached and reset it when the timer rolls over.

The trick is to recognise that both the reload and capture registers are 16 bits and although you can technically get a very wide range of PWM 65K:1from the device, in practice you are limited to nearer 10 bits or less when you want realistic square wave frequencies (more than say 40 Hz)

Hope this helps you get started

Steve