Medicare Advantage (Part C)
These are health plans sold by insurance companies but overseen by Medicare. They are alternatives to original Medicare, and usually offer more services than original Medicare.
To qualify for Medicare Advantage, you need to have Medicare Parts A and B. So that means you'll at least need to pay the Part B monthly premium.
On top of that fee, you may need to pay a monthly premium for the Medicare Advantage plan itself. The prices vary a great deal depending on the plan you've chosen. Some may have further charges, like deductibles and premiums for additional coverage for things like prescription drugs.
This is supplemental health insurance provided by private insurance companies that covers some of the costs Medicare doesn't, including copays and deductibles. You don't need, nor can you buy, a Medicare Advantage Plan if you carry a Medigap policy.
To join a private Medigap plan, you pay a monthly fee to an insurance company in addition to the premium you pay to the government for Medicare Part B.
The costs of Medigap plans vary a lot, depending on the coverage, the company, your location, and your age. So make sure to shop around before buying one.
Programs vary from state to state. Be sure to find out what your state offers. You can get help from your State Health Insurance Assistance Program. Call 800-MEDICARE to find yours.