Information from www.medicare.gov:
If you didn’t sign up for Medicare Part B when you first became eligible, you may be able to sign up during the General Enrollment Period. This period runs from January 1 through March 31 of each year. During this time, you can sign up for Medicare Part B at your local Social Security office. If you get benefits from the Railroad Retirement Board (RRB), call your local RRB office or 1-800-808-0772. Your Medicare Part B coverage will start on July 1 of the year you sign up.
Important: The cost of Medicare Part B will go up 10% for each full 12-month period that you could have had Medicare Part B but didn’t take it, except in special cases. You will have to pay this penalty as long as you have Medicare Part B.
If you already have Medicare Part A and need Part B you can sign up for Part B at your local Social Security office or by calling 1-800-772-1213 (TTY users should call 1-800-325-0778).
For more information on how delaying enrollment in Medicare Part B could result in paying higher Part B premiums, see our FAQ: What if I'm over 65 and didn't enroll in Part B during my Initial Enrollment Period?
For more information on delaying enrollment in Medicare Part B if you or your spouse is still working, see our FAQ: Can I delay my Medicare Part B enrollment without paying higher premiums? If you can delay your enrollment because you or your spouse are still working, the General Enrollment Period will not affect you until after you (or your spouse) stop working.
If you are a military retiree or the spouse or dependent child of either a military retiree or an active duty sponsor, see our FAQ: How does the Medicare Prescription Drug Improvement and Modernization Act impact enrollment in Medicare Part B for military retirees or the spouse or dependent child of either a military retiree or an active duty sponsor?
If your income is above a certain amount, then you may have to pay a higher Part B premium. For more information, see our FAQ: Medicare Part B Monthly Premiums in 2008.