Tank or tub of some kind is best for your crabbies, since it will hold in the moisture. I personally liked a mixture of sand and eco-earth for my substrate, which should be at least 1.5 times as deep as your largest crabbie is tall. More is better, as long as they still have room to climb as well, cause they do like climbing up fake vines and the like as much as they like digging through the sand.
Do yourself and your crabbies a favor and get rid of the pellets. Your crabbies will want tiny bits of fresh fruit and veggies, as well as shrimp [the dried kind you get for fish work perfectly] and occasional bits of protein like fish or chicken. Clean out old food and replace with fresh at least 1 time every day. Try different things, like flowers [not store bought or covered in pesticides] but try to avoid things like bread, because they like to store food in their shells, and that can clog up their gills. But be certain they're getting that shrimp, because it helps keep them from trying to eat each other.
You should also have two water bowls, one with fresh water, the other salt, both deep enough that they can submerge at least most of the way, but with something like vines in there to help them get back out. If the salt one is a little too salty don't worry, they regulate the salt level in their shells anyway. Do not give them baths. Some people suggest it, but it's really not good for them.
Don't be surprised if one or both dig under for more than a month at a time. Crabs molt underground, and they also love digging and tunneling, so make sure you keep the substrate moist enough that it won't fall in on them.
Heat and humidity of your tank should both be around 75-85 if I remember correctly, which shouldn't be too hard as long as you have a solid lid and under tank heater.
Don't worry if your crab likes slightly smaller shells, some do. Just make sure you've got a wide variety in many different sizes so that they have plenty of options to choose from. If there is a kind they seem to like [with purple pinchers it's normally the turbo shells they favor] get more of that kind than the others.
You might want to see if you can adopt another crab or 2, since they like buddies, and might miss their friend if their friend decides to burrow down for a while. Don't introduce them right away though, cause new crabs can have diseases. You'll want a small second tank for sick and new crabs to settle in for a few weeks so you can make sure they're healthy and not loosing legs. And yes, if they are sick or stressed they will drop legs.Don't worry too much if that happens though, they do grow back!
And in case I forgot anything in the 5+ years it's been since I had crabbies,
Forum linky! 
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