You should have been automatically enrolled in Medicare if you are currently receiving SS benefits and should have already received your Medicare card with your Medicare number. If you are receiving SS benefits and have not yet received your card, call 1-800-MEDICARE to let them know.
If you are not receiving SS benefits, you need to manually enroll in Medicare ASAP because it starts December 1 if you were born in December. You can enroll by calling the same number or enroll online at Medicare.gov.
Once you have your Medicare number, call your state Medicaid agency and ask if you can also remain on Medicaid as dual eligible. If you are currently on Medicaid through the ACA Marketplace, you may not qualify as dual eligible because the ACA only looks at income, not assets.
If you do not qualify as dual eligible, there are also "Extra Help" programs called Medicare Savings Programs. You enroll in these through the state Medicaid agency, also.
Medicare Savings Programs:
https://www.medicare.gov/your-medicare-costs/get-help-paying-costs/medicare-savings-programs