If you are currently working, you may elect to enroll only onto Part A (hospital benefit). There typically is no premium because you have paid into the system while working. You can go onto www.ssa.gov and enroll online onto Part A only. If the employer has over 20 employees, your Part A will be secondary to your employer plan.
If you work for an employer of under 20 employees, you can be asked to leave the employer plan, in which case you will also need to enroll onto Part B (outpatient services) and can do this online as well. There is a premium for Part B; it is based on your adjusted gross income filed with you tax return (example: if you turn 65 in 2013, Social Security – the administrative arm for Medicare, will be looking at your tax return for 2011).
If, however, you also want to start collecting your Social Security, you will want to call Social Security M-F 7am to 7pm @ 800-772-1213.