A cat's charm is in its independence and free spirit. Cats are very intelligent and can be trained according to ones needs. But they have their natural behavior, which is at times confusing and can cause inconvenience to others, hence a proper training is very essential.
Scratching- Cats have the tendency to scratch with their claws which removes nail sheaths and outer layer of dead cells. If they do not find a proper place to scratch then they would choose one their own. It is better to provide a scratch post to discourage your cat from scratching the furniture. The posts should be tall and covered with sisal rope. Introduce the scratching post in a very playful manner and praise them on scratching it. If they scratch some other object then say 'No' in a loud firm voice.
Litter- Place the litter tray in a quiet and clean area that is easily accessible. Make sure it is kept away from its feeding and breeding area. Wash the litter box at regular intervals with soap and warm water. Cats are clean and bury their droppings. The kittens learn such habits from their mothers and can be easily trained. There are various kinds and materials of litter trays available so get the one which best suit your cat. Also the size of the box should be according to the cat's size. If the cat is small avoid a deep litter box, and if cat is large enough then get a bigger litter tray.
Kittens are delicate and needs to be supervised constantly. They normally play with each other or their mothers. Also they like playing with moving things. If none of these are available then they come to humans. Give your kittens some fun toys like ping-pong balls or long strings but make sure they don't swallow them. Train them correctly from the start.
Cats tend to get aggressive at times, in such a case use a water spray bottle. Never hit them as it often causes a unfavorable reaction and they get all the more aggressive.
To train your cats to sit quietly or stop meowing constantly treat them with food or something they like. Keep the food in your hand and position the hand over their head. When they calm down or sit then reward them with the food.