growing a cactus..how?

cactus need sunlight.. if it's indoor, put it near the window where it can get direct sunlight.

The problem sounds like your cactus need drainage and this is the most common cause of root rot. What you need to do is replant it in a pot with medium sized stones on the pot (or smaller depending on size of your cacti), then put a cactus mix (its sandy unlik regular garden soil for drainage) then plant the cacti and you want to put sand overtop that to help the cactus to stay in place so it will not become dislodged. You can even find this fine sand in the craft section of Walmart or the salt water section of aquarium stores, even Home Depot. You NEVER want to water them more than once a week since even if you occasionally miss a watering the cacti can compensate with their own food inside their skin. Excess watering is another cause of root rot and the cacti have to have At Least 6 hours of DIRECT sunlight.

I'm no expert on the matter, but I did live in the desert for something like 11 years.
One, water the cactus no more than once or twice a MONTH. Two, make sure it gets PLENTY of sunlight. You also may consider what kind of soil you are trying to raise them in--again, I'm not expert, you might want to research all of this, but it might be the wrong kind of soil--too much or too little of certain nutrients.
Good luck!
David A. Reedy...