DavidLMO said:Keith. "Caring for". Do tell. I am a newby and have a sad looking cactus (could not resist rescue $ 19 plant for $ 3) that I am trying to care for back to health.
First, cacti that are missing all roots cannot be watered and kept out of direct sunlight. Once they start growing roots on their own then you can lightly water. I would need to see a photo to help you better.
Generally, clay pots are best for cacti as plastic holds moisture and can create rot. Most cacti love lots of warm light, indoors you rotate the pot daily so they grow evenly. During cool months (November-March here) I don't water at all unless the cactus is very small then it would need some light watering. I buy miracle gro cactus soil and add lots of perlite to it and tiny fish smooth gravel this helps drainage.