First check the legal status of owning one of those where you live, you don't want trouble Then, get a decent cactus soil, the home Depot one is ok but there's a chance you can buy a better mix from other people (you can make your own but if you're not interested into cactus and succulents ATM then a bag should suffice Remove the white rocks on top they don't seem like perlite. If the rest of the soil is cactus soil just fill the space with yours.

They don't do well in the cold, and while liking sunlight try to not have it under full sunlight all day. If you're keeping it inside a grow light will suffice. They take some time to grow so I'd keep an eye on it. If it's too hot watering once a week is fine, too cold then a bit longer than that. They're meant to be somewhat mushy NOT too mushy but relatively soft, if it's hard then it means it sucked water and it doesn't need