I'm not 100% sure, but I think close to December they will mail you paperwork and at that time you will tell them if you want Part A, Part B or both. There normally is a fee for Part B (approx $90?) but Part A is free. They automatically enroll you in Part A.
I believe Part A is for hospital visits and possibly doctor's appts.
Part B will cover your nebulized medications & some diabetic supplies.
Part D is the prescription coverage & I don't have that, so maybe someone else can advise on that and what it cost.
I would suggest that you check their website too. It is very informative.