Answer: Compared to other houseplants, cacti like to be quite "potbound", with their roots almost completely filling the pot. Repotting may not be necessary, but if it is, use a pot only slightly larger than the pot the plant is in now.
If you are sure your plants needs repotting, you can use a special potting soil developed specially for cacti. Or use any lightweight mix amended with some sand for extra drainage.
In general cactus need high light levels with at least some direct sun. They should be kept relatively dry. Starting in the spring, water the plant thoroughly once, then wait to water again until the soil has begun to dry out. Starting in mid-summer, begin reducing water so that by winter, you are giving it very little water. This way, the plant gets the rest period it needs. Fertilize just once a year, in early spring.
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