My cat knows she has a litter tray. You can buy the tray

and litter at any bit super market or pet shop.

There're different litter. So you have to choose one that suits your cat and your place. I use the paper one. It's less smelly and it's not too messy. Some ppl like those sand litter, but I dont like since normally the sand is either white or yellow in colour, and your cat will walk around your place with a white / yellow bottoms. :P And you may find those sand on your lounge too. :P:P

I also recommend you get cat litter bag from either reject shop or pet shop. It will help when you clean after their mess. :)