Enrollment

If you are getting benefits from Social Security or the Railroad Retirement Board (RRB) you will be automatically enrolled in Part A and B on 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.
If disabled you automatically get Part A and Part B after you get disability benefits from Social Security or certain disability benefits from the RRB for 24 months.
You automatically get Part A and Part B the month your disability benefits begin.
If you’re automatically enrolled, you’ll get the red, white, and blue Medicare card in the mail 3 months before your 65th birthday or your 25th month of disability.
You need to enroll for Part A and B if:
You aren’t getting Social Security or Railroad Retirement Board benefits (for instance, because you’re still working).
You qualify for Medicare because you have End-Stage Renal Disease (ESRD).
You live in Puerto Rico and want to sign up for Part B.

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 keep Part B and will pay Part B premiums.

Cost
Part A
You usually don’t pay a monthly premium for Medicare Part A (Hospital Insurance) coverage if you or your spouse paid Medicare taxes while working. This is sometimes called “premium-free Part A.”
You can get premium-free Part A at 65 if:
You already get retirement benefits from Social Security or the Railroad Retirement Board.
You’re eligible to get Social Security or Railroad benefits but haven’t filed for them yet.
You or your spouse had Medicare-covered government employment.
If you’re under 65, you can get premium-free Part A if:
You got Social Security or Railroad Retirement Board disability benefits for 24 months.
You have End-Stage Renal Disease (ESRD) and meet certain requirements.
Part B
You pay a premium each month for Medicare Part B (Medical Insurance). Most people will pay the standard premium amount. However, if your modified adjusted gross income as reported on your IRS tax return from 2 years ago is above a certain amount, you may pay more.

Most people pay the Part B premium of $99.90 each month.
You pay $140 per year for your Part B deductible.
If your modified adjusted gross income as reported on your IRS tax return from 2 years ago (the most recent tax return information provided to Social Security by the IRS) is above a certain amount, you may pay more.
Part B premiums by income

If your yearly income in 2010 was
You pay
File individual tax return
File joint tax return
 
$85,000 or less $170,000 or less $99.90
above $85,000 up to $107,000 above $170,000 up to $214,000 $139.90
above $107,000 up to $160,000 above $214,000 up to $320,000 $199.80
above $160,000 up to $214,000 above $320,000 up to $428,000 $259.70

Copayment or coinsurance amounts may apply for some services under Parts A and B.

Complete Medicare information at the official U.S. government Medicare site