You can search for brief records in the NARA Access to Archival Databases (AAD) page > Military Personnel. If you don't find the records you need, you can request Military Service Records from NARA via Standard Form 180 (SF-180).
If you are a veteran or next-of-kin of a deceased former member of the military, you may now use eVetRecs to request free copies of military records, including DD 214 / Separation Documents. You can start the form online then print and mail the form within 20 days to complete the process.
If you are doing genealogical research in military service records, please see Military and Veterans Research and Resources.
In gathering the necessary information to order records, you may find these sources helpful.