Substrate can be paper, some people use paper towels as it is easy to clean, and is cheap, however it also is not great to look at, so many people who do start useing it change pretty quick. Other alternatives are slate tiles (Easy to clean aswell and more importantly the hold the heat from an under tank heater), You can by preformed "flooring" for your vivarium from most pet stores that sell reptiles. Reptile bark chippings are handy as they are cheap and are too large to be accidentally swallowed. However if useing bark chippings or bark mulch from our garden center ALWAYS microwave it first to kill any small organism that may make your Gecko ill.
Sand should not be used, Gecko's lick everything around them, and so will swallow the sand, also when eating say a cricket they can dive at the cricket mouth open to catch it and end up taking in alot of sand, sand impaction will kill a gecko. That includes ALL types of sand even the so called safe calci sand and repti sand.
Gecko's don't like sand anyway they prefer a hard surface, they come from savanah type landscapes, hard packed dry soil and rock. Sand is not natural for them to live on.
If you do get a gecko make sure you ALWAYS have a moist hide for him in the dank, somewhere hidden, dark and with moist mosspeat for preference in it, so he can safely and easily shed.
Along with a dish of calcium and some water thats pretty much all a gecko needs.