If you are age 65 then you are probably eligible for Medicare. If you have a qualifying disability then you may also qualify for Medicare. For either case, you must be a U.S. citizen or a permanent resident of more than 5 years to qualify for Medicare once you are age 65.
If you do have a qualifying disability or illness and have been disabled for more than 24 months then you should also qualify for Medicare. The enrollment process is usually automatic, but in some cases you must enroll yourself. If you want to qualify for Medicare C or Medicare D you must first be qualified for, and enrolled in Medicare part A and B.