Britain and Germany were involved in a battleship arms race. There were concerns in Britain that Germany wanted to expand in Africa, in addition to the growing strength of the now united Germany. The Austro-Hungarian empire was also loosing power, especially over the Balkans and the Ottoman Empire was known as “The sick man of Europe” as it was failing economically. Both Germany and Britain recognised how important this could be in the future.
There was also a treaty between Russia and France to combat the growing power of Germany, whilst the British signed up to the Entente Cordiale, with France and I think Russia promising help in case of invasion. The invasion of Belgium was the straw that broke the camel’s back as far as the UK was concerned.
There is no quick answer to the question and as mentioned before and historians are still debating the issue at least a hundred years later. As more documents are still becoming available the discussions will probably continue indefinitely.
Just as an aside, many historians think that the Treaties in Paris, Versailles included, laid the seeds for WW2. A favourite question could be “to what extent was WW2 inevitable, following the outcome of WW1”
BBC bitesize would be a good place to look for the causes of WW1. Hope this helps.