Cat = loner?
Cats still have the reputation of being loners. The image of the "lonesome ranger" is stuck in the peoples' heads for many centuries. But this image isn't quite correct. There are indeed cats who do not like having other cats around (and those cats shouldn't be bothered with another kitty in the house), but usually cats appreciate feline company. Many cats enjoy social contacts and form friendships with other cats. They even mourn when they're losing a kitty friend. Cats are capable of building up well-functioning social structures if necessary. Feral cats living in big colonies to have better chances to survive are the living proof.
One cat - or more?
Especially when your cat is kept scrictly indoors - being alone all day because you're at work - you should consider to adopt a second cat. Cats who are lonely and bored can easily become depressive and lazy, and they may develop behavior issues. Boredom can be very hard on cats. So a second cat may bring more fun into kitty's life! However, having more than one cat doesn't mean that this is a substitute for playing and cuddle sessions with humans! If the cats are alone all day because their two-legged friend is at work, they will appreciate each other's company, but also enjoy the attention of their human when he's back home.
Now if you would like to adopt two cats or more, you should make sure that there's enough room for them. Even if a cat enjoys feline company, there should still be a place that provides some solitude. So it's recommended that you shouldn't have more cats than your house/appartment has rooms - one room for each cat. Of course each cat should also have its own food and water bowl, its own sleeping place, and its own litter box. Even when they're using each other's things as well, it's important to the feline soul to know it doesn't need to share if it doesn't want to.
Okay, a second cat then ...
The best solution would be to adopt two cats right from the start, best would be litter mates. They'll have the best chances to stay good friends later in life. But it's also possible to get cats adjusted to each other when they haven't met before. In most cases, this is working very well, even when the first cat has lived on his own for a while. But you should keep some things in mind for a better start!
First, when you choose your second kitty, you should make sure that its temper and character are matching with the resident cat. Usually it's no problem to compensate a few years difference in age, but a senior who only wants to enjoy the remaining years in peace would probably not be too thrilled about a lively kitten that's turning the entire place upside down. Also, if your first cat is more of the shy and timid kind, you better do not choose a self-confident, roughly playing kitty as a companion. As long as the resident cat is still relatively young, it's probably the easiest to adopt a kitten as a second cat. However, there are no strict rules here! Cats are individuals, and thus they may surprise you! You might have racked your brain to find the perfect companion for your kitty, but they end up fighting constantly. On the other hand, sometimes cats become great friends that you had never expected to like each other!
It's understood that the new cat has gotten all the required vaccinations and has been checked by a vet before moving into the new home! After all, you don't want to expose your resident cat to the dangers of a viral disease or to possible parasites!
The new kitty moves in ...
So you have found a cat you believe it could be a good friend for your kitty at home. Now you have to keep in mind that the way the newcomer is introduced will be crucial for the future relationship of the two cats. It would be wrong to just hold the new kitty into the other's one face, as if you're like "Look what I've got for you!" That way, a defense reaction of the first cat would be guaranteed. You have to be much more subtle. If possible, the new cat should stay in a seperated room during the first days. Before the cats are introduced to each other, you should show them objects the other one has been in touch with. So the cats will know each other's scent before they actually meet.
When the cats are finally introduced to each other, please don't panic when there's hissing and growling first! This is quite normal in the beginning since the cats have to work out who's the alpha cat. It's important not to interfere here. No way you're allowed to take sides or scold one of the cats! Only when one cat might get seriously injured, you should step in. Otherwise leave it to your cats to work this out. Once the ranking problems are solved, there are good chances of the cats to become frinds.
It's also understood that no cat should be prefered to the other one. Often people have the tendency to give more attention to the new kitty, once it has moved in. But you should try to avoid doing this; after all, you don't want to provoke jealousy between the two felines. The resident cat should keep the privilege of being the "first cat" of the house; which means it's still the first who's getting food, attention, etc. . These things are important to cats. It's easier for the new cat to accept this ranking; for the resident one it would be confusing and frustrating to suddenly be in the second place.
Even if your cats won't become best friends, there can be at least a more or less peaceful coexistence in which the animals are feeling comfortable. But you should be aware of the fact that it may indeed happen that two felines cannot stand each other, and that they're acting utterly aggressive. Sometimes there's no other way but re-homing one of the cats, as sad as it is. In such a case, it would be unfair to give the resident cat away since this would be most traumatizing. But in most cases, cats are appreciating feline company.