Cats are territorial and so usually do not like any new arrivals, when you bring home a new cat or kitten it is likely that there will be some disturbance while they sort out who is the boss.
It is also important to note the age of your existing cat(s) and the age of the new arrival as it is unlikely that an elderly cat will take to an energetic kitten.
Here are a few things to consider when introducing your new cat or kitten to your exisiting cat(s)
Try and introduce your new cat gradually, bring your new cat into a room just for them to get settled into their new home. Put food, water, toys and litter tray in the room.
Once your new cat has settled into their new home, start by swapping bedding or food bowls with your exisitng cat(s), this will help them to get used to each others smell.
Once your cats are used to each others smell you can start to introduce your cats face to face initially under supervision, allow the cats to smell each others territory.
Don't worry if they hiss and growl at each other, they will do this until they have established who is boss, only intervein if it starts to get nasty.
If one cat starts to get very agressive then they may not be ready to be introduced, you should seperate them and try again once they have calmed down.
The length of time it takes for your cats to get on will depend on each individual cat and their personality.