If your tortoise is the one shown in your avatar, then I'm assuming it's a redfooted tortoise?
Redfooted tortoises need to be kept in a humid environment, however, if you have the substrate too wet, they develop shell rot on their plastron quite easily. Moisten the underneath substrate by pouring water down into a corner, then cover the habitat with plastic or foil. A few potted plants inside and sunk down into the substrate also help with humidity.