I am using pcf8591 an ADC with 8051 and using 11.0591 MHz freq.
I have problem in coding, how i give clock freq and instruction to ADC, and what clock freq should I give to ADC.
The pcf8591 is controlled by a serial I²C-bus interface from the microprocessor. If your microprocessor does not have such an interface, then you will have to write code to generate the I²C protocol.
It has an internal clock, so you don't need an external one.
All this info is in the data sheet for the pcf8591. I suggest you understand it completely before continuing.