Go to medicare.gov for info - and like all Gummint stuff, it's almost mind-boggling at first.
There is an enrollment period every year, and that period is also when you can make changes if needed.
There are people who know the system, but they're not likely to be the ones calling on your phone and speaking with an accent.
Some hospitals and providers may not accept some insurers, like Humana - check with them prior to adding non-Medicare supplements.

In my case, with Medicare & the Plan G from MoH, I paid nothing for detached retina surgeries, cataract surgery, or hip replacement, except for medications.
I didn't add coverage for meds but could at approx $150/year.
Being blessed with generally very good health, that's been basically a "wash" so far.