Millions of seniors today are enrolled in Medicare. Without it, they'd no doubt have to scramble to find affordable health coverage. You may be gearing up to enroll in Medicare yourself. But if you're worried about botching the process, here are a few rules to be aware of.

1. You can sign up before your 65th birthday

You're generally eligible for Medicare coverage starting at age 65. Some people may be able to get coverage earlier due to having specific conditions.

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You don't have to wait until your 65th birthday to sign up for Medicare, though. Your initial enrollment window actually begins three months before the month of your 65th birthday, and it ends three months after that month.

So all told, you get seven months to sign up. And if you enroll a month or two after turning 65, your coverage will be retroactive dating back to your 65th birthday.

2. You can delay your enrollment without penalty if you're covered by a qualifying group health plan

There's a reason seniors are often advised to sign up for Medicare during their initial seven-month enrollment window. If you go too long without coverage, you could face lifelong surcharges for Medicare Part B. Specifically, you'll pay 10% more for Part B for each 12-month period you were eligible for coverage but didn't sign up.

But you should know that if you're covered by a qualifying group health plan through a job (either yours or your spouse's), then you won't automatically face penalties for a late Medicare enrollment. Rather, you'll get a special enrollment period that begins once you separate from your job or lose your group health coverage -- whichever happens first.

It could pay to keep a group health plan as your sole coverage if your employer's insurance offers great benefits and isn't very costly. Plus, staying on a group health plan could make it possible to continue funding an HSA (something you can't do as a Medicare enrollee).

But if you're going to delay your Medicare enrollment due to having coverage already, make sure your group plan qualifies. Generally, you're OK if your group health plan has 20 or more employees enrolled. You may want to check with your benefits coordinator at work or even contact Medicare to make sure you're entitled to a special enrollment period based on the coverage you have.

3. You can sign up for Part A alone if you don't need Part B right away

You don't have to sign up for Medicare Parts A and B simultaneously. If you're covered by a group health plan at work and you don't have an HSA, then it could pay to enroll in Medicare Part A only at 65 and sign up for Part B later on.

The reason? Part A is free and can serve as secondary insurance in the event of a hospital stay.

Enrolling in Part A alone, however, will take HSA contributions off the table for you. Make sure you're either not funding an HSA or don't mind that restriction before signing up.

There's lots to know about Medicare enrollment. So your best bet is to read up on the process well ahead of your 65th birthday. That way, you'll know exactly what to do once your initial enrollment period begins.