Medicare plans have open enrollment - it is worthwhile doing your homework before choosing to go with one of three options:

(1) Medicare alone. Part A only covers hospital costs and is usually free. Part B covers doctor bills and testing. This does not cover medications and you will have a 20% copay for physician costs. Part D covers medications and is NOT adminstered by Medicare but is managed by private insurance companies - you should make sure that you get this to avoid a penalty that will be applied forever. 

(2) Medicare plus a supplement plan. This will cover the copay and deductible of Medicare part A and B and sometimes covers part D and foreign travel (Medicare does not cover medical care outside the US, even though this is usually much cheaper). 

(3) Medicare Advantage. If you use one of these plans, you do NOT have Medicare as such - the insurer you pick will be paid by Medicare to administer your insurance. These plans have the benefit of simplicity (they often cover medications and possibly other things like glasses and hearing aids) but the disadvantage that you may be told which hospitals and doctors you can use. They are very profitable for the insurance companies concerned, so expect to be bombarded with sales offers. 

Before picking Medicare Advantage or part D plans, make sure that your medications (if any) are covered.