Training a kitten to use a scratching post.
If your kitten still has claws, the scratching post is almost as important as the litter box. If you have room in the house, you should even consider having more than one of both.
Whenever the kitten goes to scratch somewhere else, pick him up, talk to him gently while carrying him over to his scratching post, and set him there. As SOON as he "gets it" and starts scratching it, overreact with joy and pet him and tell him how good he is. It'll cause him to experience endorphins that he'll associate with the scratching post. Give him a treat for extra measure.
If after a week he's still not getting it, take his front paws in your thumb and index finger, press them GENTLY, causing his claws to pop out, and very gently drag them along his scratching post in a top-to-bottom motion. Do this four or five times with each paw slowly and gently, talking soothingly to the kitty, then let go. Then, pretend you're a kitty and scratch the post yourself, and make it clear you think you're having the time of your life doing it.
Remember: You're Mommy now, and the kitten will watch you for clues on what to do to grow up into a proper cat. When in doubt, show the cat how to do something manually.
I didn't have this problem with Diamond; his previous owners had him declawed. I personally don't believe in de-clawing cats, but it wouldn't stop me from adopting a given cat.