Part A is your hospital benefit; typically through working (or your spouse working), you have no premium for Part A.
Part B covers outpatient services (i.e., doctors, lab, physical therapy, etc.). There is a premium for Part B; it is based on the adjusted gross income on your IRS tax return. Social Security (which is the administrative arm for Medicare) typically looks 2 years back to your tax return to determine whether you will pay the minimum premium for Part B or be assessed a higher premium because your adjusted gross income was higher. (Note: if your adjusted gross income is higher, you will also pay a surcharge for your prescription drug coverage.)
If you are working for an employer that has over 20 employees, you are still expected to sign up for Part A when you turn 65, even though it will be a secondary to your employer insurance. If you are not working or work for an employer with fewer than 20 employees, you will need to sign up for Part A as well as Part B.
When you have Part A and Part B, you then can enroll on the Medicare insurance coverage that you feel will best fit your specific medical and financial needs. Please remember that being on Medicare Part A and B only leaves you with financial responsibility for the deductibles and copays not covered by Part A and B.
Part C refers to Medicare Advantage/HMO plans. Typically these plans also include a prescription drug benefit. Should you select a Medicare Advantage/HMO plan, all of your doctors will normally need to be in the same medical group and accept the HMO plan you are choosing.
Part D refers to prescription drug coverage. If, you decide you want to enroll onto a Medicare supplement/medigap policy for freedom of choice of medical providers and to fill in the deductibles & copays not covered by Medicare A and B, Medicare you will, per Medicare, need to enroll onto one of the stand-alone prescription drug plans approved by Medicare for your area. (Note: as I indicated above, many of the Part C Medicare Advantage/HMO plans already include the Medicare required prescription drug coverage.)