This can be a tricky situation since an unregulated diabetic cat is starving internally — at the cellular level — and restricting calories, in some cases, can be very dangerous.
If your cat is overweight, please see Feline Obesity but you need to understand that it is beyond the scope of a webpage to explain how to implement a safe weight loss program at the same time that you are dealing with an unregulated or newly diagnosed diabeticcat.