Well, if you wanted to do it manually, you'd need a load and trim sheet, where you can enter the weight distribution for the cargo, fuel and passenger compartments and then derive the CG, like this one:
http://formation.trafic.free.fr/A%E9roform/Trafic/Loadsheet/LS%20B734.pdf
That's for a 737-400 though - couldn't find one for the -300.
For a sim I'd just pick a random number (between 12 and 30, I think) to mix it up and pretend that's how the aircraft was loaded.