So you’re turning 65. Congratulations! It’s time to start taking advantage of your Medicare benefits. And just how do you do that?

First, here’s a quick overview of the benefits:

Medicare has four parts, A through D. Part A pays for hospitalization, and most eligible people don’t have to pay premiums for it. Part B covers doctor fees, outpatient care, home health care, and preventive screenings for cancer, glaucoma, diabetes, and other diseases. Part B has a monthly premium, which for most beneficiaries is $104.90 this year.

Part C is Medicare managed care and Part D is prescription drug coverage.

If you already get benefits from Social Security or the Railroad Retirement Board (RRB), you’ll automatically get Part A and Part B starting the first day of the month you turn 65. (If your birthday is on the first day of the month, Part A and Part B will start the first day of the prior month.)

You’ll get your red, white, and blue Medicare card in the mail three months before your 65th birthday. If you don’t want Part B, follow the instructions that come with the card, and send the card back. If you keep the card, you’ll keep Part B and pay Part B premiums.

If you aren’t getting Social Security or RRB benefits (because, for instance, you’re still working), you need to sign up for Part A and Part B.

It’s easy to do. You can sign up by calling Social Security at 1-800-772-1213. If you’re 65 or older, you can also apply online for Part A (if you don’t have to pay premiums) and Part B at www.socialsecurity.gov/retirement. The whole process can take less than 10 minutes.

You can sign up when you’re first eligible for Part B. If you’re eligible for Part B when you turn 65, you have a seven-month window that begins three months before the month you turn 65, includes the month you turn 65, and ends three months after the month you turn 65.

Sign up early! That way you’ll avoid any delay in getting your benefits. If you sign up during the first three months of your Initial Enrollment Period, in most cases your coverage starts the first day of your birth month. (If your birthday is on the first day of the month, your coverage starts on the first day of the prior month.)

If you wait until the last four months of the Initial Enrollment Period, your start date for coverage may be delayed as long as three months. You may also face a penalty in the form of a higher Part B premium.

If you didn’t enroll in Part A and/or Part B when you were first eligible because you were employed and covered under a group health plan based on that employment, you have a Special Enrollment Period. That means you can sign up any time while you or your spouse are working and you have employer or union group coverage. Or you can enroll during the eight-month period that begins after your employment ends or your group health coverage ends, whichever happens first.

Usually, you don’t pay a late enrollment penalty if you sign up during a Special Enrollment Period.

But here’s an important caveat: If you have COBRA coverage or a retiree health plan, you don’t have coverage based on current employment. You’re not eligible for a Special Enrollment Period when that coverage ends.

For more information about enrolling in Medicare, visit www.medicare.gov/MedicareEligibility. You can also get free, personalized counseling about Medicare from your state Health Insurance Assistance Program (SHIP). To get the phone number for your local SHIP, call 1-800-MEDICARE (1-800-633-4227).

David Sayen is Medicare’s regional administrator for Arizona, California, Hawaii, Nevada, and the Pacific Territories.