A damp cloth works just fine if you are trying to clean off finger print smudges and whatever else may have splashed on the screen. Microfiber cloths, like the ones they sell with iPad accessories and even for cleaning eyeglasses, are soft enough that they will not scratch the screen.
There are any number of cleaning kits and wipes that you can get that are specifically designed for cleaning electronics, most of which are located in the electronics department of any store, but I don't really bother with them.