This is a classic longer essay question. There was been long and varied debate amongst historians as to why the First World War broke out.
The standard list of reasons for the outbreak of the First World War includes: The alliance system, developments in the Balkans, the arms and naval race, a Germanic bid for World-power status and the blank cheque given to Austria-Hungary. These can simply be evaluated and one assigned a greater prominence than the others. But a higher level answer must take into consideration the historiography behind this question.
Immediately after the First World War there was a consensus that Europe had 'slithered over the brink' into war and it was no one nation in particular's fault. This cosy consensus was disrupted by a German historian Franz Fischer who argued that Germany was primarily responsible. Disrupting the existing system in a bid for world power. This led to a much wider historiographical debate amongst those who accepted Fischer's thesis and those who rejected it from the 1960s. This led to debate between those who, although they accept German guilt in starting the war, argue over whether it was a deliberate attempt to start a war, to stop their own encirclement. Or if it was a miscalculation. The first are developing the Fischer thesis, whereas the second, led by Andreas Hillgruber, are changing it in a crucial way. The centenary of the outbreak of war also led to the publication of many works. Christopher Clark harks back to Lloyd George, laying emphasis on developments in the Balkans that drew all the Great Powers towards war.
Although the density of all this work, and this is ony a portion of it, can seem very daunting it should be remembered that all of the schools are working from the same basic factors listed at the beginning of this explanation. Students need an awareness of the outline of this debate, but should avoid getting bogged down in it. Focus on the root causes and then apply them to a historiographcal context.