The CG comes defined when you enter the weight data on the FMC. After that you will see on the right botton corner the CG value, which you have to enter in the next page where the speeds are calculated (I am telling it from my memory - sorry if I forgot a step).
Once you have entered the CG value you will get the trim setting % (which usually varies betwen 5 and 5,75%). This is the value that has to be set on the cockpit before takeoff.

I hope it helps.

Nice flights !!