You have a number of options:
1. Enroll in Medicare Part B and pay the monthly premium of $ 104.90 month and enroll in a Prescription Drug Plan. With Original Medicare, you will pay the 20% of hospital and physician not paid by Medicare. You can pay a penalty is you don't have cretible coverage and don't enroll in Part B
2. Enroll in Part A and B and purchase a Medicare Supplement, which, depending upon the plan selected, will all or part of the 20% not covered by medicare. You will need a Prescription Drug Plan
3. Enroll in a "Zero" or "Low" Premium Medicare Advantage Plan. Hospital and physician care provided under a MAPD or MA plan are county based and require use of networks, which may limit access.
4. Keep your current Employer or union group policy. If you have any co-insurance, you should make a comparision of the out of pocket, benefits and cocverage to determine if making a change from a retirement plan to Medicare, with associated MAPD or supplements makes sence.