This is a common question, so we wrote an entire post on it. Plan G <--- Click this link to see more about Plan F and G.
This is such a long answer that it doesn't really fit well here in the FAQ section, so we've dedicated an entire page to it. How Much Is Medicare Part B
If you already have Medicare Part A and wish to sign up for Medicare Part B, please complete form CMS 40-B, Application for Enrollment in Medicare - Part B (Medical Insurance), and take or mail it to your local Social Security office. You can also call Social Security’s toll-free number 1-800-772-1213 (TTY 1-800-325-0778). or contact your [...]
The annual Medicare enrollment period is October 15th through December 7th, every year. During this time you may shop and replace your Medicare Advantage and Part D Prescription plans. This annual enrollment period does not apply to Medicare Supplements. If you wish to change your Medicare Supplement plan, you may do so anytime during the year, [...]