You can sign up for a Medicare prescription drug plan through Medicare.gov. The online tool will guide you through the plans that are available in your area, how well they're rated and your total annual medication costs. You're first eligible for Medicare Part D during the seven-month period around your 65th birthday, including your birthday month and the three months before and after. If you don't enroll in Medicare Part D during this initial enrollment, you'll have to pay a late enrollment fee.