When you turn 65 you automatically become eligible for Medicare Part A at no direct cost, but Part A is strictly for hospitalization and related costs. Part B is what covers outpatient care, lab and xray, etc. and if have Part B it will pay 80% of usual and customary charges, leaving you with 20% to pay directly. But be aware that 20% of a large number is still a lot of money. Those of you who have been well and fortunately not needed extensive medical care might be astonished at how costly outpatient care can be, easily running into several thousand dollars for a complex visit to the Emergency Department, or for the cost of diagnositic testing ordered by your physician (such as an MRI or CAT scan). Supplemental insurance normally covers the 20%, and you generally get what you pay for. Good coverage will be expensive since the insurance companies are not offering policies to be nice, they are looking to turn a profit. There's nothing wrong with that, just be aware that when it seems like you are getting a deal that is too good to be true its probably not true.
Another factor to consider is that Part B itself, at over $100/month deducted from your Social Security check, will be considerably more expensive based upon your overall taxable income. It has nothing to do with age or health or anything else except your income in the previous calendar year and in some cases will double or triple the monthly cost for Part B. If you are retired military you must have Part B or your Tricare for Life coverage will not be in effect.
Also be aware that some health care providers will quietly say that they will not bill you for the 20% of Part B that the government does not pay, but if they do this they are committing fraud. Should Uncle Sam find out that they are not making the effort to collect the 20% from you, they will recompute the amount Medicare pays by considering the 80% amount that they paid as now the full 100% charge. They will then reduce their payment to 80% of the 80% and you could be stuck for the difference. There's no such thing as a free lunch, so you have to make your own decision as to whether or not it is worthwhile to pay for a supplement which will cover the unpaid part of Part B.
Part D is totally separate and covers prescription drugs. However, if you choose to go with a Medicare Advantage plan offered by many of the large health insurers, typically it covers what would be covered by Part A, Part B and Part D as well. But there will be various co-pays and deductibles, as well as limitations of what physicians and hospitals you have to use with a particular plan. Its a complicated business with no absolutely right or wrong solution. The Medicare Advantage programs may involve a monthly premium for you to pay, or there might be no premium at all. And if you can't afford any of this, you might be eligible for Medicaid (which is run by each State with varying rules and limitations). If you are covered by Medicaid it will pay everything not covered by Medicare. The big negative of Medicaid is that payments are often so low to doctors and others that many health care providers will not accept Medicaid patients, so you might have coverage but not be able to actually find a doctor to take care of you.
And remember that as you age you become more likely to need the services of a nursing home at some point. Medicare only covers post-hospital rehab type skilled nursing home care, not long term care. Generally the Medicare coverage is only for 100 days maximum, and if the care you need is not deemed to be "skilled" (which has very specific requirements and not just your lay opinion that the care is indeed skilled) it will not cover you. Typically if the care can be rendered by less than a Registered Nurse it is not skilled. Paying for a nursing home privately varies greatly around the country but a good average estimate today is probably about $80,000 a year for nursing home care. If you can't afford that, you likely could be covered by Medicaid but first you would have to exhaust most or all of your savings and assets, then the State steps in. A further problem with Medicaid is that most quality nursing homes will not take you in directly on Medicaid because it pays so poorly. In many cases you would need to pay privately for a good nursing home to agree to admit you, then if the home is Medicaid certified you could or might be able to remain there when you run out of money. If the nursing home is 100% Medicaid certified (a less and less common situation) then once you are admitted, for any reason, you cannot be kicked out if you must be covered by Medicaid. If you have a living spouse in your home you might be able to retain the home and some assets while being covered by Medicaid, but legally the State could then take the home and assets after the death of the spouse to cover what it paid for you in the nursing home. Depressing, isn't it?
The best solution is to not get sick or hurt, live a long life then drop dead without any health care needed at all.