Cats aren't pack animals, so it's normal for your cat to immediately show hostility. Being territorial animals, cats also are driven to attack unfamiliar felines that enter their turf.
If you want to add a second cat, gradually introduce it to your pet over the course of a couple of weeks. During this period, house the newcomer in a separate "safe" room with food, water, a litter box and cozy hiding places. This will allow the kitten to get its bearings in its new home.
Let the cats get to know each other one sense at a time. First, they'll hear each other through the door. Next, take a sock and rub the new kitty. Deposit that sock in your resident cat's area. Rub your resident cat with the other sock and place that in the newcomer's room. This is how they'll start to become familiar with each other's scents. The next step is to allow them to switch rooms. Let each cat explore the other's area by itself.
Finally, introduce the cats to each other for brief periods while offering positive forms of distraction, such as treats or interactive toys. Try to keep the cats far apart at first. Make the sessions brief and always end them on a positive note. Bribery, with food treats, play, or catnip, is a great tool when introducing cats.
