Good information from Farmerjan above. I will add a little more about the Medicare supplement (also called Medigap) plans, especially Plan G. When it is time to "sign up, go to the Medicare.gov website and enter your zip code and sex and ALL plans offered in your state/county will be presented to you. In my area, there are 47 Plan G choices with varying premiums (G is just one of 8 plan choices). Lowest cost is $96/month. ALL of the Plan G choices are required to offer the same benefits. Those are set by Medicare. So, if all 47 of those choices offer the same benefits, why not just go with the lowest premium? There is a good reason. The lower premium choices are lower costs when you are younger, but the premiums increase as you age. Don't all the 47 choices have increasing premiums as you get older. Yes, they do. But the higher premium choices at signup will have premium increases due to inflation and cost of medical care. The lower premium choices will have those increases but will also increase due to your age. Those lower premiums choices at signup will end up being among the highest premiums as you get older - independent of inflation and cost of medical care. So, a person in poor health might select those low premiums at signup thinking they won't see much increase due to an expected shorter time on medicare. But a person in good health should probably choose a slightly higher premium at signup and avoid those age related premium increases.
Can't you change plans every year if you want to? Yes, you can. But I think you forfeit those "no premium increases due to age" when you change plans or carriers. There may also be issues with pre-existing conditions.
What is this Plan "G"? Plan G is the most comprehensive coverage. There are also Plans A, B, D, G, K, L, M, and N. With various levels of coverage. Lots of choices in plans, carriers and premiums. Buy the book "Medicare for dummies" and start reading 2 years before signup. The choices you make at signup are big decisions.