You must have both Original Medicare Part A & Part B coverage before you can enroll in a Medicare Advantage plan.
There are also only certain times that you can enroll in or make changes to a plan:
Medicare Open Enrollment: Oct. 15 – Dec. 7 each year.
Medicare Advantage Open Enrollment: Jan. 1 – March 31.
During this time, you can make fewer changes to your plan than you can during open enrollment.
Medicare Initial Enrollment: You can enroll in Advantage plans during initial enrollment. This is a seven-month period around your 65th birthday.
Special Enrollment Periods: Depending on special circumstances, you might qualify for a special enrollment period for your Advantage plan.
Medicare General Enrollment: If you enroll in Medicare Part B during General Enrollment, which occurs from Jan. 1 – March 31, you can enroll in Advantage plans from April 1 – June 30.
Certain Medicare Advantage plans are not available in all areas, and if you qualify for Medicare because you have ESRD (End-stage Renal Disease), then you might not qualify for Medicare Advantage.