It is completely possible to toilet train a cat. However, it is generally agreed upon that most cats have to be kittens at the beginning of training for it to really work. There are a number of methods for potty training cats, but here is the one that my mother used for her Maine Coon kitten (around 5 months old):
First, put the litter box right beside the toilet. Leave it here for a few days, longer if your kitten is having trouble using the litter box every time. Its important that they get that skill down first, otherwise you risk finding surprises all over your house for the rest of time.
Then, gradually raise the litter box. Put books or boxes underneath it. Phone books work great. Keep raising the litter box until the stack is the same height as the toilet seat. Over the course of a month to a week, slowly move the litter box over the seat. Move it by about an inch or two at a time.
Obviously, you can move the box when you use the bathroom yourself, but make sure to put it back right away. Otherwise, your kitten is likely to get confused.
Once you've got the box directly over the seat and your kitten is adjusted, remove the box completely, and toss some litter into the bowl. You may have to go back and forth for a bit putting the litter box on the seat if your cat has trouble adjusting.
I'm not sure if this would work with an adult cat; I've only heard of kittens taking to the practice. But that doesn't mean it isnt worth a try! I'm sure to some extent it depends on the disposition of your kitty. Good luck!