Cactuses (also called cacti) are an amazing family of plants, built to live in low-water, high-temperature climates. They are ideal for many housing situations, as they can handle a fair amount of abuse and survive or bounce back. They also have a unique quality to them that makes them stand out against most other houseplants. Even though they are hardy, proper cactus care can help your plants stay strong and healthy, flower regularly, and survive for many, many years.
After you buy a new cactus, one of the first things you'll want to do is get a new pot to serve as its home. Most have been living in their current pot for quite some time once you buy them, and have probably outgrown that living space. One important part of cactus care is making sure your plant always has enough room to grow. Get a pot that's just a bit bigger than the one you bought it in, to give it room to stretch out. Make sure to get a pot with ample drainage, though, because they can easily be overwatered and rot; this is why it's important to get a container just a bit bigger than the one the cactus was bought in, because too large a pot can lead to overwatering as well.
Another important part of cactus care is making sure the soil you're using is right for your plant. Normal potting soil is often too rich and tends to hold too much water for a cactus to really flourish, so you'll want to buy special cactus soil if you can find it. If you can't find it, you can approximate it by combining equal parts potting soil with sand, and adding some larger gravel pieces to give the soil extra drainage.
When you do move your cactus, either in the beginning or later as it grows to a larger container, be careful with how you treat it. It is during repotting that cactuses are at their most fragile, and good care means taking this into account and being gentle with them. Break the pot from around the cactus, if you can, rather than digging it up, which can cause root damage. Gently place it on a bit of new soil in the new container, and fill in the extra area with your special potting soil. Then let the plant sit out of the sun, and don't water it more than a little bit. Give it a week or so to adjust to its new setting.
Cactuses have two real seasons: an active season and a dormant season. During the active season, your cactus care regimen will be more time consuming than during the dormant season; during the dormant season you'll need to leave the plant largely alone. Water during the active season until the container is mostly saturated, with a bit of dry soil left on top. Leave it for a few days or a week until the soil is completely dry, and water it again at that time, checking occasionally to make sure the drainage is open and ample. During the dormant season, water your cactus no more than once every few weeks, and water it sparingly.