There are quite a lot of articles about toilet training cats. Good idea really as it takes away all of the messy business of "doin-the-litter". I'm very expert at that, I'm proud to say :)
There is a product out here in the UK called the Litter Kwitter. It's a training aid which gradually habituates your cat to the idea of using the human toilet.
You first put the device, which is a bit like a human toilet seat/litter tray, next to the toilet and then some time later, on the toilet (affixed to the toilet seat).
Some time thereafter that, a hole can be opened up at the bottom of the device and in time it becomes the cat's toilet.
It cost £39.99 in the UK. I have no affiliation to the manufacturer, by the way. My cat couldn't use it because being old she couldn't jump up.
There's also the issues (Toxoplasmosis) that I have discussed in another article on cat litters which are to be born in mind (click on the behavior label in the margin to see it).
Photograph reproduced under creative commons with words added by me credit and copyright raptortheangel (Flickr)