I just went through the paperwork. Medicare Part A is free. I just filled in the paper saying I didn't want any extra parts, any upgrades or the pharmaceutical program. That means I pay nothing for Medicare. I have insurance from my previous employer, so for me no more coverage is necessary. What Medicare Part A covers is hospital and expenses with a few other things. For your at-work insurance, it means that's an expense they don't have to pay.
The feds send you a card after you respond to which coverage you want which you present along with your regular insurance card.
Warning....you will be hounded to tears by insurance companies selling "gap" insurance and other types of coverage. I've found some of the reps are pretty rude and talk down to me like I'm old and senile or whatever.