Medicare is the US government's health insurance for US citizens over age 65 and those that are disabled under 65 after being on SS disability for 2 years. US Citizens pay Medicare taxes their working lifetime and qualify for "Free" or $0 premium Part A if they pay into the system for 40 quarters; spouses can qualify through their working spouse. Medicare Part B, while partially subsidized by taxes, does have a premium. Premiums vary base on your income when you enroll in Medicare.
Both Medicare A and B are obtained through the Social Security office. Part C and Part D are purchased through a broker or can be done by going direct to the insurance company that provides these benefits.
NOTE: You cannot be enrolled in Medicare C and Medicare D - you can choose one or the other for your drug coverage.
Most people that enroll in a Part D (stand-alone drug card) also purchase a Medicare Supplement to help pay the costs not covered by Medicare, including deductibles for hospitalization, outpatient services, and 20% coinsurance that is not capped.