Real 737-800 - Load Calculated

Hi,since i don't have any knowledge on how in the real 737-800 or any aircraft, all weight was calculated, example how to get(in pre flight) ZFW, Pax and cargo weight, fuel weight and overall TOW, also V speed (V1, Vr, V2 ) since all this in flight sim we can use auto calculate or atleast we put some figure, that we know the value. in flight sim also we can use Tomaflex to calculate V speed.So, how this calculate in real word.RegardsShaheedan