Not all service records from WWI survive. Of those that do survive, some are available for free. For example, those for Canada are currently being digitised and placed online by Library and Archives Canada. Fire destroyed some of the service records for the United Kingdom; those that remain are available through subscription websites such as Ancestry and findmypast. Fire was also responsible for the loss of many service records in the United States.
Fortunately, many towns, counties, schools, and companies created biographical directories to honour those who served in WWI. Local libraries often hold some of these in their collections.