I'm 64 and self-employed, and I pay for my own health insurance. I plan on taking Social Security when I'm 66. When should I get serious about applying for Medicare?

Your Medicare enrollment window opens three months before your 65th birthday and closes three months after it, but apply at least a month before turning 65. Your coverage starts a month after application.

As a self-employed person, you're likely to find Medicare costs less -- perhaps substantially less -- than you currently pay for health insurance.

There are two basic categories of Medicare: traditional Medicare and Medicare Advantage, and both involve dozens of choices.

Medicare Advantage plans are subsidized by the government, but are run by private insurers. Medicare Advantage plans cover more services than traditional Medicare -- unlike traditional Medicare, they typically include prescription drug benefits, for example -- and they usually cost less. But you're limited to network providers, and vulnerable to the insurer's business decisions. From year to year, companies can drop their Medicare plans and/or change a plan's benefit package.

With traditional Medicare, you can go to any doctor who accepts Medicare. Your hospital and doctors' services are covered, but you must buy a separate Medicare Part D policy for prescription drug coverage. You may also want to buy a Medigap supplemental policy to cover traditional Medicare's co-payments and premiums. (Medigap policies can't be used to pay for out-of-pocket expenses in Medicare Advantage plans.)

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Long Island residents can choose among a dozen Medigap policies, more than 50 prescription drug policies, and over a score of Medicare Advantage plans. You can compare them all at medicare.gov. Check each plan against a list of your doctors, the medicines you take and the pharmacies you use.

THE BOTTOM LINE Give yourself plenty of time to choose a Medicare plan.

WEBSITES WITH MORE INFORMATION medicare.gov and nwsdy.li/medicareprimer

TO ASK THE EXPERT Send questions to Ask the Expert/Act 2, Newsday Newsroom, 235 Pinelawn Rd., Melville, NY 11747-4226, or email act2@newsday.com. Include your name, address and phone number. Questions can be answered only in this column. Advice is offered as general guidance. Check with your own advisers for your specific needs.