Teelies covered the basics, you will need a tank which is at least 5 feet long and 2 and half feet wide, larger is prefferable and you will need perfect water quality 100% of the time, rays are very sensative to nitrogen pollution. Healthy happy rays should eat whatever you give them though normal foods are frozen whitebait/silver sides, prawns/shrimp, mussels and cockles. For picky rays live bloodworm, earthworms and live shrimp are usually best until they start to accept frozen foods.
As far as i know freshwater rays are illegal to import or own in Australia, i could be wrong though.