I just bought a cactus and honestly ? I'm in love. I had one when I was a child but it died, so seeing a cactus in my house again just makes me so happy, and I thought I would give you the very basics tips of taking care of a cactus because teaching is very fun !
1) You need to take it to a place with as much sun and as less humidity as possible. It's a desert plant, so it hates humidity and it likes sun ; but be careful to not expose it straight away in a place with a lot of sun, sudden changes in temperature can kill it.
2) For the types of cactus which are "multiple" (I'm talking about those kind of things) : turning it regularly (so all the sides are exposed to the sun) is adviced, so the cactus will not grow irregularly.
3) In summer, you need to give it water every 2 or 3 weeks (2/3 times a month). In winter, it's once a month. However, if you see that the ground is still wet, wait some days/weeks. Never give water to a cactus in a wet ground : it will drown in the water and die.
4) When giving water, always humidify the ground, NEVER the plant itself !
There's a simple tip to know if you are giving enough water : water it copiously (because yes, it needs water) until the water starts to escape the bottom of the plant pot. Then, stop. Simple, isn't it ?
5) If your cactus is falling over, it can be because of 4 main reasons : 1) the container your cactus is potted in is too big, 2) your cactus doesn't have enough water, 3) your cactus has too much water, 4) your cactus lacks sunlight. There are other reasons (like pests), but these are the main ones.
Hope you enjoyed that !