You are on disability right? You start out being enrolled in Medicare Part A. After a period of time they send you enrollment for Part B. You do not have to sign on for Part B, but if you did, they will remove the monthly cost from your check.
If you do not sign up for Part B when it is offered, they will charge you more later.
Good point and more information on Medicare eligibility for SSDI recipients here:
http://www.ssa.gov/disabilityresearch/wi/medicare.htm
Will a beneficiary get Medicare coverage?
Everyone eligible for Social Security Disability Insurance (SSDI) benefits is also eligible for Medicare after a 24-month qualifying period. The first 24 months of disability benefit entitlement is the waiting period for Medicare coverage. During this qualifying period for Medicare, the beneficiary may be eligible for health insurance through a former employer. The employer should be contacted for information about health insurance coverage.