Dogs and Cats Under the Same Roof
Dogs are often territorial and will fight not just new cats but other new dogs as well. They tend to mark their territory and will fight anyone who invades their space. Cats have the same tendencies and even if they are smaller than dogs, they will scratch and fight for their territory.
The first step is obedience. This is necessary especially for the dog, which is usually the aggressor in this kind of situation. You have to train your dog to obey when you say "no".
Allow your cat to wander around the house. Cats love to roam and this will make her scent scattered in your home. After doing this, lock your cat in one room and let your dog wander around the house sniffing for the cat's scent. This will make your dog acquainted with your cat's smell and train the dog that the cat is part of the household.
Dogs usually start barking whenever they see some other animals in their territory, hoping to scare them away. They won't lunge at something immediately. Once he starts barking, this is where the obedience part starts to come in. Say "no" and make him realize that the cat is also a pet.
If your dog starts to calm down when in the presence of the cat, then it's time to let him sniff the cat. Once your dog starts to decide not to bark at your cat you know that you've done the introduction part successfully.
The final step is making sure that both pets have the time to coexist. This part is usually the cat's fault. Cats tend to be aloof and stay away from the dog and other people. It's not because they are afraid of the dog but more because it's their nature to be alone.