i want to make 10k variation of resistance using parallel network of 12 resistances and switching them through microcontroller,please tell me how to calculate each resistance to get minimum step variation,switching will be done in binary(2^12=4096)combination.

I can only guess what you want

1) more words does not make you look stupid
2) are you making the equivalent of a DAC
3) an uC can not switch resistors, you need an analog switch
4) if I guess right, have a look at a digital potentiometer.

Erik