What type of items should i have in a hermit crab cage?

OK so I got my first hermit crab and I'm not really sure what type of items to put into the cage... So far I have a water dish, a large shell to put food into, a sponge for water to keep the air around him moist and the cage has like wired on the outside and he likes to climb to the top and try to get out when I am putting new food in his shell. Also I have read online that they should have sand or soil in there cage but is the rocks you would use in fish tanks OK and how will i know when he'll need to change shells or when he'll start molting?