If you work for an employer with 20 or more employees, you only need to activate your Part A (hospital benefit). There is typically no premium for your Part A which will be secondary to your employer plan.
You will want to confirm that your employer plan provides drug coverage creditable/comparable to Medicare drug coverage, keeping a copy of the page in your employer plan evidence of coverage that validates the comparable coverage. I recommend this so that should you be told at the time you sign up for a Medicare Advantage/HMO with drug coverage or a stand-alone drug plan that Medicare requires confirmation of your creditable drug coverage, you will be able to easily forward your documentation to the carrier to pass onto Medicare (*see note below).
When you are leaving your employer insurance, there are 2 forms that will then need to be completed (one that you will complete; the 2nd will be completed by your employer) and taken to Social Security to activate your Part B (outpatient) benefits before you can enroll onto a Medicare Supplement/Medigap plan or a Medicare Advantage/HMO plan.
(*Please note: If the drug coverage an individual was on is not considered creditable drug coverage, there is a penalty of 1% per month based on the national average premium of a Medicare drug plan. The penalty will be assessed back to the month you turned age 65 and you will carry that penalty in perpetuity.)