Personally I would consult the sidebar for the list of frequently asked questions. You can find an awful lot of information there. I would also consult the reading list as there are very good books on the causes of World War I.

There were many factors that led to the outbreak of World War I. It is, in my opinion, akin to throwing a match on a gasoline soaked tinder-pile. Most people attribute the cause to Franz Ferdinand's assassination by Gavrilo Princip in Sarajevo, Bosnia and Herzegovina. Ferdinand was the Archduke of Austria-Hungary and was the target of the terrorist organization the Black Hand, of which Princip was a member. On June 28, 1914 Ferdinand and his wife Sophie were assassinated while traveling to a hospital to visit people who were wounded in an earlier assassination attempt.

Alliance treaties were another reason World War I broke out when it did. It is the equivalent of two people who are about to fight bringing their friends to help them, In 1914 Austria-Hungary declared war on Serbia. This in turn brought Germany and Russia in to the war, as they would not leave their allies alone. France entered the war in response to their ally Russia entering it. And due to France entering the war, Great Britain was brought in. It was a slippery-slope.

Imperialism led to competition between the great powers of Europe and this led to increased tensions. They were constantly competing for colonies and their resources, especially in Africa.

Britain was renowned at this time for having the largest navy. And Germany sought to be their equals. This increase in militarism pushed the countries towards war.

The Slavic peoples in Bosnia and Herzegovina desired to no longer be part of Austria-Hungary, but wanted to be part of Serbia. Each country in Europe tried to prove they were the best. In a sense, nationalism was the biggest factor towards World War I breaking out.

I'm sorry if anything posted here is inaccurate or lacking. This is my first post to /r/askhistorians. I have been focusing on World War I for several weeks now and I am just eager to share my knowledge. If need be, I can supply some sources.