Speaking as an electrician and electrical contractor, there is some bad information here.
Note that Damon said light switch, not receptacle. There is no need for a neutral in a switch--but it might be there.
If you just have two wires one is hot in, one is hot out. With newer wiring you may have a ground (bare) but not in this case.
A lot of old wire has insulation the same color. And so old the insulation breaks off if you move it. Shrink tube can replace the insulation better than tape.
An induction tester can tell you which is the hot in but this knowledge is not crucial.
Newer places will have a black and white wire but there is no guarantee that the black is the hot in. If the wires are going directly from the switch to the light fixture then the white should be the hot in to the switch. Why? Because it is against the code to have a white wire bring the hot into the light fixture.
Assuming of course this is not low voltage lighting--and I recently reworked a place built in 1951 that had low voltage to the switches.
Bottom line in this case, just replace the switch.
As you can see, even something as simple as a light switch can get complicated fast. Which is why my professional services are in demand!
As an aside, you can replace an ungrounded receptacle only with another ungrounded receptacle--no U shaped hole in the front of the device. If you use a GFI in this situation you have to put on a label that says "no equipment ground." These come in the box with the GFI.
Jerry
Art and Orchids B&B
http://www.artandorchids.com