Remember, the appointment is for you not the endocrinologist.
So, if I was in your situation, the preparations I would make would be to work out what I want from the appointment: what questions do you want to ask? What addition help do you want? Have you read about any treatment that needs support from your health care team?
Apart from that, be prepared to explain how you manage your diabetes and any issues that you have.
A good endocrinologist should be willing help and not make your life difficult.