Calathea are commonly known as prayer plants because at night they curl up their leaves like two hands coming together in prayer. 
Thrives in dappled light. Bright indirect light and part shade are okay, but direct sun will scorch and dry out the leaves.
They like regular watering and to stay slightly moist. Check the soil regularly and if it is becoming dry then water. 

Like most houseplants, Calathea respond better to filtered water than hard tap water.
Calathea thrive in humid environments so place in a bathroom or kitchen, or mist every so often. 

Crisping curling leaves: humidity too low, exposed to direct sun. 

Dropping leaves: not enough water.