The Tank
Glass or plastic tanks can be used for hermit crabs. A 10 gallon aquarium with a lid (a sliding glass one will work well to contain humidity) is a good choice. Plastic tanks with lids are now readily available in a variety of sizes and the larger ones of these can be used the mesh lids (don't retain humidity well) make them less optimal for long term housing. The small plastic homes with accessories sold as hermit crab kits are too small. These plastic cages make excellent temporary homes or isolation tanks though. As for minimum size there are varied opinions, but a general rule is that the larger the cage or tank, the better. I'd recommend the equivalent of a 10 gallon tank as a good size though, as it allows room for the cage furnishings needed and offers enough space for a few crabs. Despite their name hermits are quite social and are best kept in groups.
The Substrate
Sand is generally the substrate of choice for hermit crabs, although there are a couple of other options. Sand is nice because the crabs like to burrow down into their substrate and sand provides a good outlet for this natural behavior. Playground sand (found at home improvement stores) works well and is inexpensive. You may want to rinse, dry, and bake the sand (at 300 F) to sterilize it, and it can be re-washed and used again. Aquarium sand is fine as well. Calcium based sands are nice and come a wide array of colors, but are expensive.
Other options include fibre bedding made for reptiles such as the coconut fiber-based bedding called Forest Bedding. The fiber is ground quite find so it is nearly like soil, so it is good for burrowing. Crushed coral is also a nice choice, but you may want to provide an area with Forest Bedding or sand as your hermit crabs may prefer these substrates for molting. Avoid the use of gravel or wood shavings.