If you bring a new cat into the house, then your existing cat may perceive this as a threat. So for at least the first few days you should try to keep them separate. Keep the new cat in a room with a closed door. It will feel safe and secure in a place where there are no other cats and the shock of a new feline friend will be minimised for your existing cat.
Then mix up their scents. So take an old towel and gently stroke the new cat with it. Then leave this where your other cat can smell it. Do the same with your existing cat and leave the towel where the new cat can smell it.
Then gradually start to introduce them by leaving the door of the room where the new cat is homed and gradually increase the time that the door is left open, to ensure they meet each other for longer and longer periods. Then they should be familiar with each other and hopefully able to co-exist happily !