You have the opportunity in the 4th month preceding your birthday month to enroll onto Medicare Part A and Part B to be effective the 1st of your birthday month. Medicare Part A is hospital coverage, Part B is outpatient services; however, since Medicare was never meant to cover you entirely, you have to options you can select regarding coverage under Medicare.
Your choices for coverage to fill in the gaps once you have your Medicare Part A and B in place are (1) Add a Medicare supplement/medigap policy to cover the medical copays not fully covered by Medicare along with adding a stand-alone drug plan (the later is known as Part D) /or/ (2) Select a Medicare Advantage/HMO plan (also known as Part C) that will then cover your medical and prescription drug needs.
You can elect either coverage (1) or coverage (2) in the 3 months preceding your birthday month and your coverage will then be effective the 1st of your birthday month.