What bloodwork has been done so far? What are the results?
What diet is kitty getting? Is kitty currently being treated with any medications?
Medications will vary with each case, but the diet is key along with proper and timely blood tests such as fructosamine - scheduling and any medications will need to be determined by your vet, not online.
I've had good results managing diabetic cats using Purina DM diet(only available from vets) as the sole diet.