Most people only pay their Part D premium. If you don't sign up for Part D when you're first eligible, you may have to pay a Part D late enrollment penalty.
If your modified adjusted gross income is above a certain amount, you may pay a Part D income-related monthly adjustment amount (Part D IRMAA). Medicare uses the modified adjusted gross income reported on your IRS tax return from 2 years ago (the most recent tax return information provided to Social Security by the IRS). You'll pay the Part D IRMAA amount in addition to your monthly plan premium, and this extra amount is paid directly to Medicare, not to your plan.