What do I need to take to Social Security when I apply for Medicare?

Instead of listing a bunch of stuff here and leaving something out, there are two resources that will give you all this information. One is the Medicare website itself (medicare.gov), and the second is a call to your Medicare and/or Social Security office/s. They will tell you everything you need to take with you.

If this is for regular Medicare because you are turning 65, they will send you a letter I think 3 months before your 65th birthday.  Since you have internet access, go to medicare.gov and click on general enrollment and eligibility for Medicare (not the part D drug plan).  They have a few questions for you to answer and then the system tells you what is needed.  You usually don't have to go down to the office since there are millions applying for regular Medicare.  If this is for Medicare for a health issue or if you're under 65, you should call the local office and make an appointment and tell you what you need to bring and ask why you're applying.  They also may send you a letter first if that is the case and let you know what is needed.  There are several scenarios per the medicare.gov web site:

The Centers for Medicare & Medicaid Services (CMS) administers Medicare, the nation's largest health insurance program, which covers nearly 40 million Americans. Medicare is a Health Insurance Program for people age 65 or older, some disabled people under age 65, and people of all ages with End-Stage Renal Disease (permanent kidney failure treated with dialysis or a transplant).

This section of the site is designed to provide you with information about your Medicare eligibility and enrollment. You will be asked to answer a series of questions. The number of questions, as well as the questions themselves will be dynamic based on your answers. Once we have collected enough information, we will display detailed information tailored to your specific situation. Please note that Social Security will make the final decision on your eligibility and enrollment status.

If you are not a U.S. citizen or a lawfully admitted alien who has lived in the U.S. continuously for a five-year period, please contact Social Security at 1-800-772-1213 for your Medicare enrollment and eligibility.

Please Note:  If you are a widow or widower, please contact Social Security at 1-800-772-1213 for further assistance.

You can do a lot related to getting the ball rolling online, but unless things have changed since I applied for Medicare (2004), at some point, you will have to appear at the SS office in your county for a face-to-face interview. For me, that is they also had me submit all my paperwork (much of it printed from what I did online), so everything could be taken care of in one trip.