The U.S. government, through the National Archives, has made millions of enlistment records from World War II available online. These are incredibly valuable for genealogical research, for filling in some gaps in family history, or simply for knowing more about what people in your family were doing back in the days of the "Greatest Generation" veterans. Here is how to get WWII enlistment records for free.
NARA, short for the National Archives and Records Administration, a US government agency, is a good place to check for World War II enlistment records online. NARA has an online service called AAD, or Access to Archival Databases. This is your best starting point, as you can find the links here not only to WWII enlistment records, but to other databases that may also be of interest. For instance, AAD includes Prisoner of War records and Naval Intelligence Personnel from the WWII era, and also links to records from other wars, from the Civil War to Vietnam, and on to the Cold War era.
There are two files for WWII enlistments: a large file with general enlistment records, and a smaller file for the Reserve Corps in World War. You can search either file individually, or search both files at once, to quickly go through the more than 9 million records that are available here.
You can conduct a simple name search of the enlistment records, but this will not always turn up the record you're hoping for. There are many possible reasons for this...it helps to read the "frequently asked questions" section provided by AAD to understand the limits of the system. It also helps to have as much detail as possible about the veteran you are searching for. Where did they enlist? What date? How old were they? To the extent such information is readily available, it can help you sort through search results, and identify the correct records.
If an initial search turns up empty, try using name variations and wildcards, such as a % for a single character or * for any number of characters, to improve search results. But keep in mind that enlistment records, though comprehensive, are not 100 percent complete due to errors and missing information as original records were being scanned into the NARA system.