Big John is pretty much correct about signing up for Medicare at your 65th birthday, but I believe its a six-month window that's 3-months before and three months after 65. You can sign up for the basics (Parts A and C), then add on PArts B and D for an additional cost. OR . . you can sign up for the MEdicare 'Advantage' program, which is basically an HMO. And make sure you sign up during that window and dont be late; severe penalties are imposed if you sign up late!!!!
If you have insurance through a big company right now call them up and see if they participate in Medicare, then ask them if they have advisors to help you through the 'mess'. The advisors are supposed to be independent, but most will try and sell you to sign up with their company as the provider, especially if you choose the Advantage HMO-style platform. For me, I'm in an HMO stlye system and they do participate in Medicare, I like the service, so I'll probably just continue on with the company under Medicare oversite.
Check out the details on the Medicare site, as they can change and I tend to spout a lot of BS anyway.