Cute kitty! Advice I've heard in the past with regard to bad breath in cats is to have the cat to the vet first. Sometimes different health problems show up as bad breath, so that's a good first step.
Second reason to see the vet? They're usually pros at teaching how to brush cat's teeth. I've no idea how to do it, but I do know it takes specialized toothpaste, and probably a whole lot of patience on the owner's part.
Best of luck!