If your cat does not have at least one scratching post, get one or two. This should solve most of the inappropriate scratching problems. Ideally, each cat in the household should have his own scratching post, and it should be in a place he can easily find it.
If you have a scratching post, examine it. Is it worn or all scratched out? If so, it is time for a new post. If the post is new and your cat does not want to use it, rub some catnip along the sides and top.
This will attract him to the post, and he will associate the pleasant scent of catnip with scratching. You also can dangle toys off the top of the post or place treats at the top. The more appealing the scratching post is, the more your cat will use it.
If you catch him scratching something he should not, correct him with a loud "No" and clap your hands. This will startle him and he will stop what he is doing. Pick your cat up and move him to the scratching post.
Place his front paws on the post and make scratching movements. If he does not seem to understand, run your fingernails down the post and pretend to scratch the post yourself. Praise your cat whenever he uses the post, and give him a treat afterward. He will make the connection that when he scratches the post he gets rewarded.
To prevent your cat from scratching things when you are not home, cover the edges or corners of the furniture with plastic strips or double sided sticky tape. Pet product manufacturers sell transparent, non toxic strips or double sided tape designed to repel cats without harming them.
Declawing is not an appropriate solution for a scratching problem. Pet supply stores sell little plastic caps that fir over a cat's nails. You can also trim the tips of your cat's nails every few weeks, which will prevent him from clawing anything.