ZFW would normally be on the flight plan doc.
Empty weight + cargo and cattle.
Fuel weight would be entered as well on the flight plan.

Weights, like routes can and do change, so they are done for each trip in flight prep, with a final check against the manifest before sign off.

Treat it like the route, a part of the paperwork. The Fuel/Cargo load managers should have that info for you, so note it down whilst loading up.

Either ZFW or GRW need to be manually calculated and manually entered. There is way forthe FMC to automatically populate this. (The other will auto populate once one is entered).

Entry is in thousands of pounds eg 12500 pounds  would be 12.5 etc.
However you can set the FMC to be metric, so check that setting and change it or adjust accordingly.

Note: GRW can exceed MTOW, as the extra fuel can be used for startup and taxi.
As long as you are at or below MTOW before you start your takeoff you are ok.