The causes of WW1 are complex at first glance. But really the key thing is to realise that there was simply too much tension by 1914 for a war not to happen.
By 1914, each country was ready for war. They had armies, navies, plans to fight and allies who would support them. Worse than that, each country had decided not to back down again. For example, when Germany was humiliated in the two Moroccan Crises and left with 100,000 square miles of worthless swamp in the Congo, they swore never to back down again. Similarly, when Russia was threatened by Germany to not support Serbia in 1908, they swore never to let Germany threaten them again.
So when Franz Ferdinand was murdered in 1914, his death could have just been a tragedy on its own, but it was the tensions of the other countries that turned it into the trigger for the First World War.
Check out the powerpoint below for simple revision notes. You will need to add detail from your revision guides and class notes, but this gives a bit of an overview.