What you need to know
First World War – a wonderful source for information, primary sources, war posters, and timelines.
Trenches on the Web – A great overall source for information about WWI
World War I Document Archive – An international collection of documents from World War
Woodrow Wilson: Prophet of Peace – Information about Woodrow Wilson and his steps toward ending World War I.
The Aerodrome – Aces and Aircrafts of WWI – Information about the planes and pilots of WWI
WWI History – from the BBC, an overview of the war
The Great War – from PBS. A very nice timeline and other information and images.
World War I Records – from the National Archives. Draft cards, images, and more.