Determine your eligibility to enroll. If you are enrolled in, or are able to enroll in, Medicare Parts A or B, you are eligible for Part D. You will usually be eligible for Medicare if you are a U.S. citizen (or have been a permanent legal resident for 5 years) aged 65 years or older. You will also be eligible if you are under 65 but are disabled, get regular dialysis for permanent kidney failure, or have Amyotrophic Lateral Sclerosis (ALS).
Enroll in Part D even if you do not currently take any type of medication. That way you will already be covered if your health changes and you do need medication, as is the case with nearly everyone at some point.
Think about what level of cost, convenience, and coverage are necessary for you when selecting your plan. Take into account the fact that you will be paying a monthly premium, a deductible, and a co-payment for each prescription, so you'll have to balance the cost of a higher deductible versus a lower monthly premium and/or a lower copay, or vice-versa.
Select a plan that covers any medications you are already taking. Not every drug is covered by every plan, so if you are taking a drug not on your plan's list, you will have to pay full price for that drug or switch to a different drug.
Take into account the coverage gap, or "donut hole", that occurs once you and your plan have together paid out more than a certain amount (currently $2,510) in a calendar year for prescriptions. At this point, you will have to pay 100\% of all drug costs until you reach yet another milestone (currently $4,050 for your out-of-pocket expenses or $5,726.25 total paid out by you and by your plan) before your drug costs are covered again.