Most people are automatically eligible for Medicare when they turn age 65. If you are a US citizen, have lived in the US at least 5 years and have worked in the Social Security system for at least 10 years you are most likely eligible for Medicare.
You may also qualify for Medicare if you are under age 65 and meet the definition of Social Security disability.
Don't confuse Medicare with Medicaid. Medicare as has been stated is an almost automatic eligibility when you turn 65 or are on Social Security Disability. Medicaid is something you have to qualify for. Many people get the two confused. You qualify for Medicaid if you meet certain income and asset requirements. If you are low income, call your Social Security office or the Department of Human Services and ask how to apply. Even if you are not eligible for full Medicaid, there may be other help available, such as extra help with prescription costs.
You may also qualify for Medicare if you are under age 65 and meet the definition of Social Security disability.