Connect the ground wire from the light fixture to the ground wire in the ceiling box.
Connect the ground wire in wall to the base of fixture
Connect the ground wire to the metal box.
The fixture will not be grounded. As long as the hot and neutral are connected the light will work.
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.
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.
Connect the white and black wires back on to where the other black and white wires connected. There should also be a green pigtail coming out of the fixture which will be the ground. If it has a chain, then the wire running through the chain will be the ground wire.
Connect them in parallel. The black wire from the ballast to the black line wire. The white wire from the ballast to the line neutral. Connect the ground wire to the ground screw which is located in the body of the fixture. Grounding of the fixtures is essential.
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.
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.
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.