If you are leaving your employer insurance, in order to have Medicare insurance coverage you will need both Part A (hospital coverage) and Part B (outpatient services coverage) in order to enroll onto either a Medicare supplement or a Medicare Advantage/HMO plan. If you are over 65 and delayed your Part B, then you will need to submit a form to Social Security verifying loss of employer coverage. Your Part B premium will be based on the adjusted gross income filed with your tax return (note: Social Security does a 2 year look back on your tax returns).
The Medicare supplements (defined by Medicare) allow for freedom of choice regarding your providers. You would also enroll onto a Medicare approved prescription drug plan that most cost effectively cover your specific needs. If you are already on an HMO, then you would choose on the Medicare Advantage HMO plans (which cover your medical and prescription drugs) that are offered, depending on which medical group/physicians you want to keep as well as ensuring that the specific drugs you take are covered by the plan.