Why is my first Medicare bill so high?

There might be a multitude of explanations.

If not subtracted straight from your Social Security payments, Medicare typically issues invoices for a period covering three months.
Recipients have financial obligations that include, but aren’t limited to, regular premiums, annual deductibles, and shared payment costs or co-pays.

A large segment of the Medicare population receives Social Security benefits, and it’s customary for their Medicare premiums to be subtracted from their Social Security disbursements each month. However, for those who are not yet retirees, these payments need to be made directly to Medicare.

Furthermore, if you fall into what Medicare designates as a higher income bracket, your contribution toward Medicare might be higher.

Have You Postponed Enrollment in Medicare?
Higher monthly premiums might be the result of penalties for enrolling late. These penalties for late enrollment come into play if you did not sign up in a timely manner for Original Medicare (Medicare Part A and Part B) or Medicare Part D.
These fines are appended to your Medicare Premium Invoice, and this could be the reason your initial Medicare invoice was more substantial than anticipated. You might reduce these penalties by opting into a Special Enrollment Period (SEP).