It is a little long, but I hope it helps.
Let me start out by saying they can be difficult to keep. They have lots of specific needs, but I'm sure you'll do a great job caring for yours since you've been doing your research.
I'm going to give you all the info I can remember off the top of my head.
Hermit crabs are social. They need at least two. Depending on what species you have, they'll need different space accommodations, as they grow to different sizes. Some can reach six inches in diameter. Do you know what species you will get? What size tank is it in?
They need 6 inches plus of substrate. They love to dig and also need to molt regulary. Some people use coconut soil (EcoEarth), some use un-dyed playsand from the hardware store, and others use a mix of both. Please, never use the sand marketed for hermit crabs. It does not retain moisture well, and can harden, trapping them when they're molting. Gravel is also not a good choice because it is difficult for them to dig in and can hurt them. I have half sand and half eco earth, and my hermits love it. They're always digging tunnels.
The substrate should be wetted with dechlorinated water when first added to the tank. Not sopping wet, just moist, so it's easier for them to dig.
When they go down to molt, it's tempting to want to check on them. Absolutely do not disturb them when molting, as this can be fatal. All you can do do is wait. They can take a long time, so long that you begin to worry, but never go digging.
All hermit crabs are wild caught, and they've been through a lot on their way from the wild to the pet store to you. They may have not been able to molt for a while due to poor conditions, so that might be the first thing they do. They also are likely stressed and weakened. If they don't make it through their first molt, don't feel bad. All you can do is give them a good habitat and healthy food when you bring them home, and then wait. I have never had a hermit die on me due to giving them great nutrition and leaving them alone to molt, but also I was just lucky that my babies held up under all that stress.
After their first molt, they should be doing much better. They'll molt continuously for the rest of their lives (it's how they grow), but the first molt in captivity is always the worst.
They need both fresh and salt water. However, the ZooMed brand saltwater marketed for them isn't the best (it has added dyes, not enough of certain minerals, etc.). You'd be much better served buying a small box of Instant Ocean or another brand of marine saltwater mix (the kind used in saltwater fish tanks). For Instant Ocean, you mix in 1 and 1/2 teaspoons of the salt for each cup of water.
They also absolutely need their water to be dechlorinated. You don't need anything fancy, just regular aqaurium dechlorinator (the kind used for fish). Otherwise the heavy metals burn them (just like with fish).
Their water dishes should be deep enough so that they can fully submerge, but not too deep so that they'll drown. I use Fluker's Groovy Lagoon, as it has steps that allow them to easily climb in and out. They need to be able to go under the water as they regulate the salinity of the water inside their shells by doing so.
They need a constant humidity of at least 80%; they have modified gills and cannot breathe without this humidity. You can buy a mister and manually spray the tank daily, or you can consult the link at the end of my post for other ideas. I have my screen lid covered so that the moisture stays in, and I haven't had to do anything extra to maintain it.
All species should be kept at 80 or higher, though different species have different needs. Between 80 and 90 degrees is a good range, and if you can keep the temperature stable, that's best. Heat pads that stick on the side of the tank are the best way to create stable heat. The best brand by far is Ultratherm. You can order one online and attach it to the side of the tank.
It's important to buy a thermometer and hygrometer to monitor humidity and temperature levels.
For nutrition, they do need a calcium source. Giving them a cuttlebone from the bird section of the pet store will allow them to access calcium when needed.
On the site I showed you, there is a nutrition guide. Please check that out. The store bought crab food has many harmful preservatives, so it's better to stick with natural/organic things you have at home. However, be aware some foods are poisonous to them (there is a list of unsafe foods on the site as well). They love fresh fruit and veggies, oatmeal and honey are favorites with mine. Make sure to provide sources of protein; if they don't get enough protein they can turn to eating each other. Mine like dried crickets, shrimp, and mealworms. Sources of fat are important, too, like coconut oil or natural peanut butter. Other good foods include dried, unsalted seaweed, unsalted nuts, chia seeds, hard boiled eggs, and quinoa.
They also love climbing, so some plastic plants and reptile grapevine are great for them. Some kinds of wood are not safe for them (like pine wood), but reptile grapevine is 100% safe. Mine really enjoy their cocohut (hollowed out coconut with a ladder made for birds), so I'd recommend getting one of those. You can also make a cave out of a jar; stick the jar about halfway down in the substrate on its side, fill it with a small layer of substrate, and then cover it. Mine sometimes hide in there, as it's dark and warm. Recycled fish net is also great for climbing.
You also need larger shells than the one they are in for them to change into as they grow. Do not get the painted shells, the paint is toxic and if they injest flakes of it, they will die. If you can, buy crabs that are in natural shells at the store. However, if your crab comes in a painted shell, just provide lots of shells for him to look at and hope he will change. You need around five shells per crab to minimize fights. There's a guide to good shells for each species on the website I provided. You can order them online or even buy them at craft stores.
I hope this helps! Again, I think I covered almost everything, but if you have any questions feel free to p.m. me.
Here is the link:
HCA: Hermit Crab Association • Index page
