Medicare has a seven-month window around your 65th birthday. Here's what to do, roughly in order. Most of it takes an afternoon.
Your job's insurance pays first, so you can usually delay Part B (and its premium) without a penalty. When the job or coverage ends, you get an 8-month Special Enrollment Period to sign up. Many people still take premium-free Part A.
Medicare usually pays first, so you generally need Parts A and B at 65. Ask your benefits office in writing, and keep their answer.
COBRA and retiree coverage don't count as coverage from current work. If that's what you'll have, sign up for Part B during your window.
10% added to your Part B premium for each full 12 months you could have had Part B and didn't, for as long as you have Part B (unless you had coverage from current work).
If you go 63 days or more without Part D or other drug coverage that's as good ("creditable"), a penalty is added to your drug premium for as long as you have Part D. Your employer or union can tell you if your coverage is creditable.
Free counselors at PA MEDI will go through this with you: find them here.