Introducing a new pet into your cats world should be done slowly and carefully to prevent conflicts from arising. Create a peaceful environment prior to introducing your cat to the new pet. Cats need time to adjust to change and familiarize themselves with the new smell that has entered their domain.
The new cat/dog should be confined to one or two rooms in the house with their own bed, food and water to allow the cat to investigate the newcomer. Gradually introduce them, by keeping the dog on leash or allowing the cats to see each other. Then work up to have them meet face-to-face with owner supervision.