There are many litter materials available, the best is the one a cat willingly uses. materials range form economic wood and paper chips to expensive dolomite varieties - based on usage, the best may be the fine, granular "clumping litter". This material is also easy to clean, as the results of a cat's toilelting are absorbed, clumped and easily isolated from the whole tray.
To encourage a new kittern ot use its litter tray, it is a good idea to place a kitten that is just waking up directly into the tray. Instinct will often take over as it moves its paws around the litter material.. Once your kitten has used a tray once, it will instinctively try to "over-mark" its own toilet scents.
It is wise to change litter material on a daily basis but young kittens will not usually produce large amounts of urine and feces so you can clean the trays after a few days.
Cats are very clean animas. to maintain an adult cat in good health, it is important that its litter train is always kept clean.