For the centennial of the U.S. entering WWI, NARA launched a new portal page for WWI Research.
The Genealogy Resources takes you to a page where you can start Researching Individuals in WWI Records. (Keep that in mind, and return to it after your tour of this page.)
Keep scrolling to find a section where you can choose from several World War I Topics.
Scroll even farther down for a World War I Timeline, and More Resources.
Explore the page and resources.
When you are ready to start your own research, a good place to start is NARA’s webpage Researching Individuals in WWI Records. A good resource is an article from NARA’s Prolog publication: An Overview of Records at the National Archives Relating to Military Service.