To get Parts A and B of Medicare, all you have to do is submit an application. In some cases, this may not even be necessary, as some Medicare beneficiaries are automatically accepted into the program. In most cases, whether or not you are automatically enrolled will depend on whether you have already started collecting Social Security benefits.
For example, people who are receiving Social Security or Railroad Retirement Board benefits at least 4 months before their 65th birthday will not have to sign up for Original Medicare. Similarly, people who qualify due to a disability will not need to sign up for Medicare, as they will be automatically enrolled once they collect disability benefits from Social Security for at least 24 months.