Connect the ground wire to the metal box.
Connect the ground wire in wall to the base of fixture
you can ground it to the fixture electrical box.
The fixture will not be grounded. As long as the hot and neutral are connected the light will work.
The green wire on the light fixture is a ground wire. If there is no ground wire in the conduit, the green wire should be attached to the metal box with a screw.
If you mean 2 bare copper wires those are the ground wires. Tie them together and then connect the light fixture ground wire which will be green or bare copper to those ground wires.
I always connect the ground wire to the box. However, if there is no real ground wire running back to the panel it really does you no good. The NEC requires that you rewire the circuit with the proper conductors. Remember that the ground is for your protection. Without a proper ground, you run the risk of injury or death if the fixture wiring fails.
If you are referring to a porcelain or plastic ceiling light fixture most do not have a place to connect the ground wire. You cannot ground these type fixtures. Just connect the ground wire to the mounting crossbar and forget about connecting it to the light itself. It is on the ceiling and will never be touched unless you change the bulb and then you will have the switch in the off position. It is perfectly safe.
The fixture box should have a ground screw on the bottom of the box. Sometimes you have to move other wires out of the way to see it. Just reconnect the fixture ground wire to this screw.
By earth wire I am assuming you mean the bare wire and that a black and white wire are connected to the light. If when you connect the earth wire and breaker trips then there is a short between black and earth. It could be a bad ground connection, an internal short in the light fixture where black wire "hot" is connected to metal on the fixture through a nick in wire.
Very often the ground wire in the fixture is ignored, or just connected to the box, if there isn't a conductor to connect to. This, however, is a code violation as any fixture with a ground wire is required to have it properly connected to an equipment grounding conductor back to the panel. This is for YOUR SAFETY. Technically you should rewire the circuit with the proper conductors. It is BAD PRACTICE to connect the ground wire to the neutral or white wire because this could create a hazard of its own.
A ground wire is a typically uninsulated copper wire. The intent is to connect the wire to an electrical appliance or fixture to provide electricity a path to ground in the event of a short, thus preventing electrocution.