Ok, your light fixture doesn't require a specific polarity so it will work either way, but it's good practice to wire it with correct polarity. Some of the newer CFL
bulbs require correct polarity for reliable operation. The two coated wires are the hot and neutral, the bare wire is ground. In your ceiling box, you should have a black (hot), white (neutral), and bare copper (ground). The two coated wires would connect to the hot and neutral, and the ground to ground.
To get the polarity right, you'll need a voltmeter that tests for continuity. A continuity test determines if the circuit is continuous. If you look at one of the light sockets, you'll see the little prong in the bottom center of the socket, this should be "hot" (black wire), and you'll see the threaded outer rim that the bulb screws in to, this should be "neutral" (white wire). What you do with the test is put one lead on one of the wires and then the other lead on the little prong (hot). If the circuit is continuous and you have continuity, that means you have the wire is wired to that little prong. If it's not, then try the other wire, you should have continuity with it. You can pick up a cheap digital multimeter at your local Home Depot or Lowes for about $20.
Let me know if you need more help.