Cats are extremely territorial so you should be wary if you already have a cat in the house and you are thinking of getting another cat. In fact, most people would choose not to get a second cat. However, this does not really mean that no household would be harmonious when there are two cats under the same roof. Still, if you want to get rid of the nasty hissing bouts that would inevitably happen, there are things you can do to foster a more peaceful environment.
The key here is in the introduction stage. Yes, you will have to introduce your new cat to your first cat very carefully. Doing this the right way would make things smoother in the end. However, it is not going to be easy so you have to be patient with the results here.
The first thing you have to do is enlist the help of a friend. Your friend should be the one to carry the new cat into the house, preferably in some sort of cat carrier so that it would not be too alarming a sight for your first cat. But why get a friend to do this? Your old cat would not be appreciative of seeing you holding another cat in your arms. So, you have to get a friend to do this for you initially.
Once you, your friend, and your new cat are inside the house already, make sure to take the new cat into a separate room. Do not attempt to have both cats inside the same room at this stage. You are asking for trouble if you would do this. In this separate room, make sure the new cat has its own sleeping place, a litter box, feeding dish, water dish, playthings, and most importantly, its own scratching post. With its separate room, your first cat will soon notice that there is indeed another cat holding its own territory in the same house.
Cats' sense of smell is very intense so you can expect your old kitty to notice the presence of the new cat by its scent even beyond those closed doors. The key here is to let your kitty smell the new cat in an indirect manner. You can do this by rubbing the new cat with a cloth or towel and then have your first cat smell that material. Occasionally, your old cat will become acclaimed to this new smell.
After some time, you should then swap the rooms of the cats so that each cat can sniff each other's presence out, making both of them more accustomed to each other as well. Occasionally, allow the cats to spend time together in the same room. This may take a few tries, but be patient. If this is still not enough, then you can enlist the services of your veterinarian. This way, you can maintain a peaceful and harmonious environment for both of your beloved felines.