There is something wonderful about that little puffball of fluff that's caught your eye and you want very much to take it home with you. Then you think to yourself, 'what about my other little puffball of fluff?'
Cat relationships with each other can vary quite a bit for a variety of reasons. Let's take a look at some of the most common considerations that should be running through your mind before your adoption.
Consider first, the age difference. If your cat is much older, and you bring a kitten into the household, the older cat may seem threatened, possibly intimidated. If the cat you choose to adopt is about the same age, there will be a higher likelihood of comparability in that department, but that doesn't mean that it will be an automatic match.
Gender should also be a primary factor in your selection, and with gender issues you have to take some things into your head. In a household with two male cats, you may be more likely to have territorial issues, especially if one is neutered and the other not. Tomcats fighting each other is nothing you want to have to deal with at three o'clock in the morning. The male/female dynamic can work at times, depending again on both animals being neutered. I've always found it best that if I have an existing female cat, I go with another female. Girls seem to bond better.
Behavior of your older cat should also be of prime importance. Is your older cat sedentary? A kitten here could disrupt that balance pretty quick. If your older cat is very active then the newer animal should be active as well, with the hope that they will play together some. If your older animal is a lap-cat, and you bring home a kitten that will require attention and the older animal may become jealous.
When two animals are introduced to each other for the first time, often times they both will become hostile. A little trick that I picked up from a veterinarian friend of mine is to keep the newcomer in a box/carrier for a while and let the older animal smell. This keeps the newcomer safe, and the old timer understand that there's another feline in the household.
It is also highly advisable that at least from the beginning that feeding locations should be separate. Animals of all kinds feel threats regarding their food. While most owners have free feed available, consider a small dish of dry for each animal.
Another good thing to do, especially for the newcomer is to have somewhere for them to retreat to. If, for example, you used a carrier to bring the new fluff into the household, be sure to leave the carrier's door open enough to allow the animal somewhere safe to flee to.
A good practice is to immediately introduce the new animal to the litter box. Ideally they will defecate and/or urinate in it. This does two things. It allows your new animal to scent mark some territory and also to let it know where the box is. Cats are incredibly easy to litter box train.
In time, the odds are highly probably that the two animals will get along, but this is a process and relationship they have to develop with one another. While you need to be a referee, the cats need to do the real work themselves.