Under the car, between the fuel pump and the injector pump.
In the fuel tank.
According to the 2000 Ford Focus Owner Guide : ( 50.0 litres / 13.2 U.S. gallons )
4 to 6 pis
35 - 55 psi
Probably the fuel filter. Change it.
56 liters
Do NOT use Diesel fuel in a Ford Focus. The engine will stop running and you will have to drain the gas tank to rid the car of the diesel fuel. Gasoline engines are not designed to run on diesel. They do not have enough compression to cause the diesel fuel to combust The only engine used in a 2000 Ford Focus was the 2.0 which is NOT a diesel engine.
Drop the gas tank.
Fuel filter.
By the right front corner of the fuel tank, up in a recess.
50 liters ( 13.2 U.S. gallons )
On the 2000 Ford Focus it is in the right front passengers footwell , behind the kick panel ( there is an access hole )