Cats need to scratch, and have various reasons for doing it: One is to take care of and file their nails to sharpen them and also remove old layers. Another is to mark their territory visually and by smell. The visual part is evident even to the eyes of humans and the smell part is to warn other cats that that is their territory, as they use the pads of their feet -where they have sweat glands- to leave their scent on the place they've chosen as their own. It's natural for them, which is why you shouldn't tell your cat off, but rather re-educate them.
OBSERVE YOUR CAT AND GET HIM AN OBJECT HE WILL LIKE
If you don't want your cat to choose an object or piece of furniture to scratch you need to provide one or several substitutes. They should have porous surfaces so your cat's nails can latch on and so they can get rid of old layers of nail.
To find out what type of object he likes, first watch him to see what type of texture he likes and even look at the areas he normally scratches: see if they are vertical or horizontal surfaces, if they are soft or rough, if he scratches around the entrance to the house or next to the sofa... When you have finished your study, buy him a similar scratching post and put it next to the piece of furniture he likes to scratch. You can gradually move the scratching post to where you want it to be. Make sure the scratching post is good quality and won't fall over.
One trick is to rub the scratching post with nepeta cataria, also known as catnip, catswort and catmint, amongst other names. This plant with a minty aroma that attracts and intoxicates felines is a good way to attract them to the new scratch post.
If you cat is already an adult you can take his paws and show him the scratching movement, mainly to get his scent on the new object and to explicitly tell him that this object is ok to scratch.
COVER THE PIECES OF FURNITURE YOUR CAT ALREADY SCRATCHES
To avoid him scratching the areas he's already marked, cover the pieces of furniture he's already scratched with a different texture (sheets, plastic, etc.). Your cat will take some time to forget his old scratching area, at least two weeks to a month.
CUT YOUR CAT'S NAILS
You need to trim your cat's nails every two to three weeks, to keep them healthy and so any scratches you get will be less painful. Be careful not to cut them too short, each nail has a vein inside that mustn't be cut as this would be painful and bleed.
You also need to create a safe space so he doesn't see these nail care sessions as something unpleasant. Stroke him, calm him and don't take longer than 10 minutes.
To find out how to cut your cat's nails correctly, ask your vet.