It is hard to understand the ins and outs of Medicare but here are a couple of simple points.
Each Medicare “Part” has its own enrollment period, late enrollment penalty, deductibles and premiums. If you are already receiving Social Security benefits you will most likely automatically be enrolled in Medicare Parts A and B. If not, you must enroll by contacting Medicare. Medicare Eligibility - if older than age 65 (the common age for receiving Medicare)
You are a US citizen or have a resident visa.
You qualify under your spouse’s or former spouse’s work record and he/she is at least 62 and: 1. you’ve been married for at least 1 year; or 2. you were married for at least ten years and you haven’t remarried before age 60.
Medicare Eligibility - if younger than age 65 (and meet certain other requirements)
You are receiving Social Security and Disability benefits for at least 24 months.
You have permanent kidney failure (End Stage Renal Disease) or ALS.