You can enroll in Medicare and any supplemental Medicare plans when you file for retirement or disability benefits (about three months before you turn 65). You can also just apply for Medicare and supplemental plans separately online at www.socialsecurity.gov or at an appointment at your local Social Security office. You can contact the Social Security Administration at 1-800-772-1213. Do apply within three months of your 65th birthday (no matter what) or you may have difficulty purchasing it in the future (not to mention it may be more expensive).
For more information about enrolling in a Medigap policy, contact your State Health Insurance Assistance Program (SHIP), which provides free education and counseling about Medicare services and programs.