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. You would not expect to pay for Part A, but there will be a premium for Part B premium that 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 in order to determine your premium for Part B which is based on your adjusted gross income).
If you are looking for freedom of choice regarding your providers, you may want to review the Medicare supplements (defined by Medicare) to see which one will fit your medical and financial requirements. In addition, you will also need to enroll onto a Medicare approved prescription drug plan that will cost effectively cover your specific prescription drug needs. Both the Medicare supplement and the stand-alone drug plans have premiums.
If you are already on an HMO, then you may want to consider the Medicare Advantage HMO plans in your area that are accepted by your medical group. An HMO will cover both your medical and prescription drugs. Depending on where you live, there may not be a premium for an HMO in your area.