Prednisone is a corticosteroid and it is not given to diabetics because it elevates blood sugar level - i. e. induces hyperglycemia. Diabetics should also avoid skin creams and eye drops containing corticosteroid group of drugs.
Prednisone is not a medicine for diabetes. Usually you would take a medicine like insulin, metformin, or glucophague. It will depend on what type of diabetes that you have on what medicine you need to take.