The First time I heard Peggy Sue

Getting old. If you know about Medicare, please leave me a comment. Since I am still working (half time) I have not applied for social security. But I will soon be age- eligible for SS regardless of my work. I have heard that if you take SS, you MUST also have medicare. I have heard that medicare doesn't really cover everything. How does that work if you have other (really good) health insurance? If you know this from experience or can point me to some good clear answers, I would be grateful.