When Britain declared war on Germany on August 4, 1914, Canada was also at war. The first Canadian contingent of soldiers set sail for England on October 3, 1914. Canadian troops entered battle on January 6, 1915.
The Canadian Encyclopedia has a detailed timeline of Canada's involvement in the war.
The Great War's YouTube channel covers the week by week history of the war exactly 100 years ago. It uploads a video every week and goes over what happened in that week during the war. There are three episodes on Europe leading up to the war, the first on the central powers, the second on the problems in the Balkans, and the last one on the assassination of Archduke Franz Ferdinand. There is also a special on Canada during the war.