When do I apply for Medicare? I will be 65 in July 2021.

Medicare.Gov will be the site that will give you all the information. You can also request a free copy of "Medicare & You 2021" the official US Government Medicare Handbook. This is free. Call 1-800-Medicare for questions. Address to request book is US Dept of Health and Human Services, Centers for Medicare and Medicaid Service, 7500 Security Blvd., Baltimore, MD 21244-1850.
Also go to the top of the AgingCare Timeline and access Care Topics. You will find many questions regarding medicare, and the answers. Good luck.

Socisl Security offices are not open for in person visits due to Covid. If you're not already on SS you have a 7 month period to apply...the month you turn 65 and the thre months before and after. You can do this online or by phone.

If you are still working for a large employer, your employer policy will likely be first payor.  You may want Medicare part A, but it is complicated.

Are you already collecting SS, if so Medicare is automatic. And you automatically receive A & B. If you are waiting till you are 66 to get 10O% of your SS then you need to apply a couple of months before ur 65. I would not wait to get part B. I think there are penalties if you wait too long. If u are not collecting, you will need to pay the part B out of pocket. Once u receive SS it will be deducted from your check.

I advise you to go to your local SS office and sit down with one of the reps. They can help you apply and answer any questions and advise you. You will need Part D for your prescriptions unless you get insurance thru work. You will also need a suppliment. Be careful about Medicare Advantage policies. They sound good but have restrictions. Medicare has contracted them to handle Medicare. With Medicare you can get any doctor you want as long as they take medicare. Not so with a MA. They have network doctors and your present doctor may not be in that network.
Your Office of Aging would be where to go to get info on what insurances are allowed to practice in your state and help you pick the one best suited to you. With my husband, his Union pays for his supplimental, Dental, Prescriptions and vision.

If you mean Medicare part A, I think 3 (or maybe 2) months before you turn 65. SS sent a letter telling me I needed to. If you mean part B, it's after you turn 65 and don't have a job with insurance (I'm waiting until I'm 66, so I won't apply for B until then), and I think you have to apply for part D at the same time, unless you sign up for part C and it includes drug coverage.

I hope that helps without confusing you. Maybe someone else has a better way to explain it.