How do i take care of cactus house plants? they keep dying. what kind of dirt? how much water? whats wrong?

It is mainly do to the type of soil mixture you have. It should be about 80 to 90% sandy medium. It's best to have them in clay pots as clay pots let the roots breathe and wicks the moisture out keeping the roots from rotting. Plastic pots keep the soil wetter and the soil can't breathe. Water your plants sparingly about twice a month. Make sure it can drain and keep it in in a bright to sunny location. I had a friend who set her weather tool bar for Arizona weather and if it rained there she would water her cactus. Your cactus will take in the moisture and then shut down because it can store water for a long period of time as it doesn't lose moisture through it's stem. If you have to re pot it a little secret is to take a news paper take a few pages and roll it up and wrap it around the cactus as a handle so you can lift it with out get stabbed. This is the plants way of keeping desert animals away from getting eaten for water.