Claws are a cat's number one tool. They are crucial when it comes to hunting, escaping, fighting and just daily survival. It is deeply instinctual for cats to keep their claws at their sharpest and in tip top shape at all times. Their lives (all 9 of them) depend upon it. In the wild, a cat will use trees or logs to sharpen and help shed the outer horny layers of their claws.
In the home, a cat will, of course zero in on your favorite piece of furniture to gleefully dig those claws into to sharpen. You probably would rather that antique sofa NOT become a very expensive scratching post. How do you train your cat to not sharpen his claws? Well, you can't. It is nature. The best you can do is to encourage him to WANT to use an object of YOUR choosing.
There many types of commercial scratching posts you can purchase. From inexpensive little cardboard boxes to huge second mortgage requiring cat condominium towers, your choices are extensive. Keep in mind that just because you find something aesthetically pleasing and priced right does NOT mean that Kitty finds it awe inspiring. Most scratching posts are made of wood and either carpet pieces or coiled rope.
To make that object more appealing than the couch, try rubbing cat nip leaves on the scratching post. You can buy cat nip or even cat nip scented spray at your local pet store and in some grocery stores. Never try to force your cat to use the new post as he will most likely develop a "you're not the boss of me attitude". By putting the cat nip scent on the post and perhaps flicking a toy or string over it to entice your cat to "dig in" you are able to make it more attractive.
At the same time, you will want to quell the siren song of the items you don't want scratched or shredded. One of the best cat deterrents would be double sided sticky tape. Cats tend to hate the feel of it. Yes, do cover the entire area of the object the cat likes to scratch with the tape. You might also try citrus scented spray or one of the animal repellent sprays available in your pet store. Just remember that with patience and persistence, you and you dear feline friend can live in harmony with your furniture and his claws all intact.