If you have a relative that served in the US Armed Forces after 1917, you may be able to get ahold of their service records fairly easily. The veteran and their next of kin need only apply at eVetRecs. Once you fill out a simple online request form, you'll be asked to provide a signature verification that must be either mailed or faxed to the National Personnel Records Center in St. Louis. If you are not next of kin, I'd suggest asking the next of kin to submit the request, if they are not available, the procedure you must follow is explained at the National Archives site: Access to Military Records by the General Public, including genealogists who are not next-of-kin.
It is important to note that in 1973 there was a fire in the military personnel records facility that destroyed many records. The affected records are described in this excerpt from a description on the National Archives website:
A fire on July 12, 1973, left the top floor of the military personnel records facility in ruins. This floor had contained some 22 million personnel folders, filed alphabetically, for U.S. Army personnel discharged from 1912 through 1959 and of the U.S. Air Force discharged from September 1947 through 1963. At the time of the fire, one-third of the air force records already had been relocated and thus saved, but overall, fewer than 4 million records were recovered, either entirely or with as little as one identifiable document.
Records of military service from before 1917 are available and can be ordered online, visit this National Archives page for more info.