Because keeping and caring for animals appropriately is a topic very close to my heart, I have summarized the most important points for proper cat care:
Unfortunately it is still a common misconception that cats are loners. They do hunt alone; but for a healthy development, both physically and mentally, they need conspecifics.
Especially with stray cats we can clearly observe that they live together in colonies, play and cuddle together and groom each other.
That's why cats should never be adopted alone. No other animal and no human, no matter how close the relationship, can replace another cat.
Even if many cats that do live alone seem happy and healthy - they would always do better with a friend! Especially since possible consequences like hyperactivity, aggression, depression and other behavioral disorders will often not be recognized by inexperienced pet parents.
Only cats that are allowed to freely go outside and explore are an exception because they will usually have interactions with other cats there. Please be aware of this though: For safety reasons cats should only be allowed outside once they are 9-12 months old. However especially kittens should absolutely never be kept alone, so in this case it is highly advised to adopt a somewhat older cat that can be let outside right from the start.
If it's not possible to let the cat outside and it absolutely has to be one single cat, there is following option as a compromise:
Adopting an older cat, for example out of an animal shelter, that was separated from its mother too early and wasn't socialized properly. Those cats usually grew up alone and are now no longer able to live with other cats.
Finding food of high quality for cats can be tricky in the beginning.
The wet food should contain at least 70-80% actual meat and there shouldn't be any sugar in it. Preservatives, flavor enhancers or milk products also have no place in cat food.
Dry food is a bit of an issue of dispute. A little dry food at times, especially as tooth care, is sometimes advised. However it shouldn't be the main food.
Most common, cheap feeds that are available in every supermarket unfortunately equal our "junk food". Specialist shops or online stores however offer a great amount of good options. Recommended are for example following brands: MAC's, MjaMjAM, Iams, Grau and more. It's good to still read through the ingredients though, since not every sort of a brand is always of the same quality. It's important to also check if it says "complete feed" or "complementary feed" on the packaging.
There is usually also a suggested feeding amount on the package. It's important not to feed the cats too much or offer treats too frequently as a lot of cats suffer from being overweight. It strains their joints and bones and can cause other health problems.
And: Offer water or cat milk (for kittens), never cow milk!
There should be at least as many litter boxes as there are cats in a household. Usually it's even advised to have one extra litter box, so that means 3 litter boxes for 2 cats, 4 litter boxes for 3 cats, etc.
Cats are very clean animals and that's why their litter boxes need regular cleaning too: remove clumps daily and depending on the litter type and box size renew it completely every 2-4 weeks.
PLANTS & OTHER DANGERS
When having plants at home it is important to research beforehand whether or not they are poisonous for cats. A lot of cats like chewing on stems or leaves, especially when they are alone and get bored.
Some of the poisonous plants are for example: lilies, tulips, jonquils, gum and dragon trees as well as the Christmas flower.
Safe plants are (among others): cat grass, catnip, valerian, bamboo, calatheas and kentia palms.
Cables and other wires or cords can also be nibbled on or sometimes cats can become entangled in them. That's why they should be protected or not within the animals reach.
Small plastic parts or plastic bags also shouldn't be lying around. Cats love to play with them or even nibble on them and could choke.
Even some cat toys are not suitable for unsupervised use since parts can be bitten off and swallowed.
More things to consider:
Can the cats escape while entering or leaving the home and get in danger? Is the balcony safe for them? After cooking, are hot stove tops accessible to them? With very young or curious cats it can also be advisable to close the toilet lid...
CLAW CARE AND GROOMING
Cats usually care for their claws on their own. To do so, they need to scratch. Aren't there enough scratching opportunities around, the furniture might suffer. That's why it is good to have at least one scratcher in every room. The options vary from cat trees or condos, hanging or laying scratching boards and posts to even some specific cat scratching toys.
Trimming the cat's claws is usually NOT necessary as they do so themselves.
Please note that declawing cats is an amputation and should never be done!
Regular grooming, especially with long-haired cats or during changes of coat is advised. Actual washing on the other hand is not necessary, with the exception of sick or very old cats that are not grooming themselves.
Having a good vet is inevitable. Even healthy cats should visit a vet around once a year (outdoor cats possibly more often) for a general checkup and their vaccination and deworming.
In addition it is important to always observe the animals, check eyes, nose and ears for discharge and the fur for parasites. When noticing any changes in behavior it can be crucial to react quickly.
Pet parents should also try to avoid stress for their animals as much as possible. Depending on the cat's character even frequent guests or loud noises (music, TV, etc.) can cause stress. Change is another big stressor for most cats, so for example rearranging the furniture a lot should be avoided too.
LYING AND HIDING
It's important for cats to have enough safe havens. Every cat should be able to retreat on its own. Ideally they can choose between actual hiding spots, like on a blanket under the bed, cat caves etc. and comfortable open lying spots in different heights.
Since cats love observing things from above, they often prefer higher places to sit or lie down. Therefore a floor-to-ceiling cat tree is a must. Additionally a cat walk or at least some boards and baskets up on the wall are a great option.
Lastly it's good to offer enough space on windowsills as cats love sitting there, enjoying the sun and watching the outside world.
PLAYING & CUDDLING
Even though cats sleep or doze most of the day, they still need attention too. While most pet parents love to cuddle with their animals, sadly playing with them often isn't a priority. However to satisfy their urge for hunting and provide enough exercise, playing with them is absolutely important and shouldn't be neglected.
Clicker training is also a great way to keep cats busy and exercise them mentally too.
While cuddling with cats it's nice to give them ones full attention while doing so. So remember to turn off the TV or set that book aside every once in a while when the little fur friend comes along, purring and asking for pets
Vise versa it is important to also respect a cats boundaries and don't force any pets or cuddles on them while they're not feeling like it.