Summarize:

Your cat will always want to scratch on things, and if it doesn't know how to use a scratching post, it will use your furniture instead. Before bringing your cat inside, place a scratching post in a dry spot, near your cat's food. Give your cat at least a week to become accustomed to using a scratching post before bringing it inside. Before bringing your cat indoors, place a little box outside in a dry location. Fill the litter box with fine-grain, clumping litter. Give your cat approximately a week to get accustomed to the litter box before bringing it inside. Make sure to clean the litter box every day. Outdoor cats tend to like to use clean areas to eliminate waste. If the litter box is not clean, they are likely not to use it. Put the litter box in a safe and quiet area. Otherwise, the cat might be too scared or hesitant to use it. Before bringing your cat indoors, you should be sure that it won't be bringing disease with it. If your cat hasn't already been microchipped, a process that allows it to be identified, your veterinarian should perform this procedure as well. A wild cat will also need to be vaccinated and neutered. Before the cat is spayed or neutered, the vet should perform a test for Feline Leukemia Virus (FeLV). FeLV is highly infectious between cats and can be deadly if untreated. The vet will do two blood tests to see if your cat has this disease. The vet should do a complete check-up of the cat. This includes checking for ear mites, fleas, lice, and other parasites. The cat should also be dewormed.
Train your cat to use the scratching post outside. Train your cat to use a litter box outside. Take your cat to a veterinarian.