Cats (in particular kittens) will frequently scratch their human companions without intending any harm particularly when playing or if given a fright. If panicked, a cat will often claw their way to freedom even if this causes (largely unintentional) damage to the person who is holding them or on whom they are sitting. No matter how safe a cat's environment is, they retain a strong sense of self preservation and can easily be startled. When playing with toys, cats and kittens will often fail to distinguish between the toy and the hand holding it and so care should be taken if you do not want to get scratched. It is also worth remembering that even the tiniest kitten is fairly hardy and human skin is very fragile by comparison. It can take a while for a kitten to learn that her human friend is so easily damaged and you should help her understand this by reacting (but not overreacting) to any scratch by saying no firmly and withholding your attention for a short period. Most cats will learn to be more careful and to keep their claws under wraps when playing but the occasional accident is still fairly common.
If you are bitten or scratched by your cat or kitten it is possible that you will develop Cat Scratch Fever (also known as cat scratch disease).
Cats need to sharpen their claws and remove the husk of old claws and so will often scratch furniture and carpets if you have not provided them with a suitable alternative. In order to avoid this damage you should buy a good quality scratching post or provide your cat with a homemade alternative (such as a piece of old carpet fixed securely around a table leg).
You will need to teach your cat to use this scratching post by holding their paws gently against the post and replicating a scratching motion. You can also make sure the cat sees you (or another cat) scratching the scratching post - although this often results in the cat giving you the kind of look that implies he thinks you are very strange! Cat nip can also help encourage your cat to use a scratching post (the spray on type is the best for this). Once your cat has the idea, remember to discourage her scratching anything else and lift her over to the scratching post every time you see her trying to scratch your furniture.
If all else fails, you can try using citrus oil to protect your furniture as cats are not fond of their smell. However, you have to be careful not to damage any furnishings with the oil.