Claws that are too long can curl back into a cat's paw pads and injure the foot (not to mention damage furniture and carpets) so indoor cats need to have their nails clipped regularly.
A scratching post by itself won't stop claws from getting too long.
Outdoor cats
Cats allowed outside need longer claws to help them climb, defend themselves and get themselves out of trouble.
Their claws wear down naturally from their more active lifestyles, so trimming an outdoor cat's nails isn't generally necessary (but keep an eye on them anyway, just in case).
Handle your cat's paws early on
Cats resist having their paws touched, so get your cat used to this while he's still young by handling his feet often.
You could do this as part of a regular grooming session - stroke the feet and squeeze the toes so that the nails extend outwards.
Build up gradually until you can handle all four feet in one session.
Bribery can help! Reward your pet with a treat after each foot-handling session.
While you're doing this, look at each claw. See the pinkish section of nail? This is called the quick, and the pink line is a vein. Don't cut this part! Damaging it would hurt the cat and make the claw bleed.
The transparent tip of the claw is the part you can clip – but only cut the first 1 to 2 mm (one sixteenth of a inch). If in doubt, less is better.
Clipping cat claws – how to do it
Invest in a pair of proper claw clippers
Hold the paw firmly and press on the base of the nails to extend them
Clip the nails downwards (shown below), not from side-to-side. Clipping sideways makes the claw more likely to split
Work quickly and confidently. If you're nervous, your pet will be nervous
Practice by cutting just one or two claws the first few times
Build up to clipping all the claws on one paw
Increase until you can clip all four paws in one session