Here is what you will need to properly house and care for a pet land hermit crab.
You will need some kind of housing. A glass ten gallon tank with a cover (and locks for the cover to prevent escape!) is a great option for housing one to two hermit crabs. You can also use cheaper plastic tanks but these do not keep the environment at the proper humidity as well. Wire and plastic cages (sort of like a hamster cage) are also sold for hermit crabs, but these do not keep the environment as warm or as humid as the crab needs to stay healthy. Glass tanks are definitely the best option.
Inside the tank you need a fairly thick layer of substrate such as sand or coconut fiber or moss bedding. The easiest and most affordable to use is sand. The hermit crabs also love the sand. They like to dig and burrow in it, and it is gentle enough that it shouldn't harm them when they molt. You can also buy calcium sand (hermit crabs need a lot of calcium) but it is more expensive then regular sand.
You will need things for the hermit crab to climb on and hide in. A coconut shell crab hut is a great place for crabs to hide in, and you can buy them at most pet stores. Fake plants, drift wood, and decorations for reptiles all provide good things for hermit crabs to climb.
You will need a heater to keep the hermit crabs environment in a temperature range of 72- 80 degrees. Anything less than that and the crab can become sick. An under tank heater is the best option. It is easy to use and pretty affordable. In addition to the heater, you will need a thermometer so that you can monitor the temperature levels.
Hermit crabs need to be in an environment of around 70 to 80% humidity (they actually breathe through gills so they need to have fairly moist air to breathe or they will suffocate). You will need a mister (sort of like a spray bottle) which you can purchase at pet stores. You can also buy sea sponges at pet stores to soak and put in the habitat to increase the humidity. You will also need a hygrometer to measure the humidity.
In the habitat you will need a few shells of various sizes for the hermit crab to change into. Three shells in addition to the one being used are a good amount. Make sure to have a smaller sized, a medium sized, and a large one.
You will need a food bowl and food which is specially made for hermit crabs. You can also buy hermit crab treats at many pet stores.
You will need to water bowls deep enough for the hermit crab to soak itself in, with a ramp or step structure to allow the crab to get out and not drown. There are usually plenty of these at any pet store that sell hermit crab products.
Since the hermit crabs need both salt and freshwater, you will need to buy some marine salt to make saltwater with. Only use marine salt. NEVER use table salt, it is not the kind of salt that they need to survive. Marine salt is sold at any pet store that sells fish supplies. Any water that hermit crabs are given needs to be free of chlorine. (chlorine will kill them). If you have city water, then you have chlorine in it, and will need to buy water conditioner found at the pet store.
You can also provide small toys for hermit crabs to play with. Such as little balls made for hermit crabs.
If you do not use calcium sand, then you need to buy calcium blocks and supplements. Most pet stores that sell hermit crab supplies have them.
And I think I covered it all!
Note: If you have more than one hermit crab, they can live together but you will need another tank or two (it can be cheap and it doesn't have to be big) for isolation. The reason why is that when hermit crabs molt they need to be isolated from the other crabs. When they molt they are very vulnerable and delicate and can be hurt or killed very easily. So when they molt they must be put in their own temporary tank.(you will obviously also need a heater, bowls, sand, and a decoration or two, along with a thermometer and hygrometer for these extra tanks too.) (so owning one hermit crab is much cheaper than two or more.)
Another note: All the information in this post is for land hermit crabs not marine hermit crabs. Marine hermit crabs live in saltwater fish tanks, (in water obviously). They are very different from land hermit crabs.