DD-214
(NOTE: you're very welcome to call us and step through it on the phone if you'd like some assistance. It will ultimately need your signature but we can arrange that remotely if required).
Go to the National Personnel Records Center link here:
https://www.archives.gov/veterans/military-service-records
Where it says: Looking for?
DD-214 / Separation Documents
...click the blue button and go through the screens, filling as many details as you can. These are fairly straightforward and ask for name, SSN, address, length and type of service etc.
On the step where it asks you what type of records choose the option "Any and all military service records" (or words to that effect).
At the end the application pop-up window will generate a form for you to print out & sign. Then fax/mail that off per the instructions.
Allow 90 days to get a response.