it costs you nothing. You do have to pay a deductible and coinsurance (usually about 20% of the amount Medicare allows) if you need care. But, it will get you in the door if you get sick and need to be admitted to a hospital.
Part B -- which primarily covers doctors care, outside the hospital or when you are in the hospital -- costs you about $110 a month. If you have no other health coverage -- like through an employer -- you should sign up for it too.
Supplemental insurance, you don't have to take. But, if you can afford it, it will pay part, or all, of your deductible and coinsurance (the 20% Medicare doesn't pay on most hospital and doctor care). The more you pay per month for this, the more of the deductible and 20% it pays. If you can afford it and have assets to protect if you get sick, it's probably a good idea.
If you have few assets, like money in the bank, MediCAID might be an option in your state to cover deductible and the 20%. Depends on your income and medical expenses.
Traditional Medicare does not cover prescription drugs, which can be very costly if you are sick. Again, if you don't have a lot of assets, Medicaid may cover you for this, but you have to see if you meet requirements in your state. But, if you can afford it, Part D (which covers part of prescription drugs) is a good idea, I think.
Right now, I am still employed. So, I signed up for Part A. I might sign up for Part B if I get sick or lose my employment.
This is a gross simplification. There are a lot of ramifications and considerations. For example, you may be able to sign up for Part B or Part D only during certain times of the year. If you put it off until you get sick, there may be penalties (again, it depends on several factors).
But, no matter what you do or your circumstances, sign up for Part A.
If you have no health coverage and can afford the $110 a month, sign up for Part B.
Talk to someone about supplemental coverage and drug coverage. Try to find some senior/elderly non-profit advisory organizations in your area.
If nothing is available, I'd try AARP. They offer some supplemental coverage, so they might suggest that, but they won't beat you up to sign up. Their counselors offer a lot of help.
Ask questions here, and good luck. It ain't easy.
Again, this is a quick suggestion on a complex topic that varies depending upon your circumstances.