The Medicare website https://www.medicare.gov/default.aspx
provides the details, but it contains so much information that it can be difficult to navigate. Here is the very least you need to know.
Part A, => no premium, is hospital insurance. has a deductible of $1,156 that covers hospital stays up to 60 days, copayments of $289/ day for days 61-90, and copayments of $578 a day for days 91-150 days.
Part B => optional insurance that covers doctors' bills, labs & outpatient care. The basic premium is $99.90/ month (can be as high as $319.70 for an individual earning > $214,000 annually), and deductible is $140/ year. Copays are 20% of Medicare-approved amounts.
Part D, => covers prescription drugs, has a monthly base premium of $32.34 (high-income consumers pay more), in addition to a premium which varies by a plan. Copayments and deductibles also vary by plan.
If you receive Social Security, you will be enrolled automatically in parts A & B when you turn 65. If you aren't yet receiving Social Security, you have to apply for Medicare, (can do online)
The enrollment period for Part B and D begins three months before you turn 65 and lasts seven months. If you miss this enrollment window, your coverage will be delayed and your premiums will be higher.