if the fluid is low, try looking at the power steering pump and see if you can see an external leak anywhere.
If you can't see a leak then look down by the rack and pinio and check to see if there is fluid leaking. If you don't find any leaks, chances are the rack and pinion seals are leaking fluid int o the bellows boots. the bellows boots are the rubber/plastic boots coming off the rack and pinion that go to the tie rod ends. They fill with fluid and eventually they will start to leak on to the floor.