Wheat Stone bridge balancing
Can you please guide how do I get ZERO when no load is applied.


Getting exactly zero is impossible. However, there exists such things as bridge balancing resistors to get pretty darn close. Thermo Fisher Scientific has a nice description of the technique starting on page 126 of their Strain Gage Based Tranducers manual. Alternatively, you can calibrate the offset out, more on that in a minute.


  Secondly, how do I adjust the milli volt output at the full load (for
  e.g I need 3mV/V at 20Kg load)?


You don't say whether this is an off the shelf transducer or something you made, but think about this for a minute. You're sensing strain. Strain is a mechanical quantity. To get a set output from some given bridge setup, you would have to change the geometry of the transducer. This is not trivial and there are people that spend their entire careers doing this. The easier thing to do is to characterize your transducer in the application. It's pretty hard to get around having a calibration step unless you really know what you're doing.