There are a number of ways to get a copy:
1. The first and best option is to request it online here. Do note that it takes about 10 days for it to be mailed to you.
2. You could also mail SF 180, a Request Pertaining to Military Records, which can be mailed to the National Personnel Records Center (NPRC) at Military Personnel Records, 9700 Page Avenue, St. Louis, MO 6312-5100. If it is needed immediately you can also fax the form to 314-801-9195 or 314-801-0764.
3. If the file is at a Records Management Center the veteran can write to RMC at PO Box 5020, St. Louis, MO 63115, or call (888) 533-4558.
4. Veterans can also write to the local Regional Offices.