Part 1: War Breaks Out
The event that started the outbreak of WWI was when the Archduke of Austria-Hungary, Franz Ferdinand, was assinated.
It starts with the Archduke taking a visit to Sarajevo in June 1914, which is now part of Bosnia and Herzegovina. His route was published in advance. Austria-Hungary had many ethnic minorities within it's borders being opressed, and with nationalism well and alive in Europe, this was going to get messy.
Some assassins threw bombs at his car, but they missed and blew up officers behind him. He goes into hiding. Then he goes back to visit the injured officers in the hospital. The driver takes a wrong turn and by sheer coincidence finds himself behind one of the failed assassins, and the Archduke is killed.
Austria-Hungary wants to declare war on Serbia, so they go to their ally Germany. Germany assures that they will help if it comes to war. Austria-Hungary send a list of 10 impossible demands to Serbia. Serbia tries to resist war. They accept every demand except one, which would allow Austrian police to operate in Serbia. If they were to accept this demand it would basically be giving up Serbia's sovereignty. Austria-Hungary and Germany declare war on Serbia.
Serbia's protected by Russia, who's friends with France, so they all declare war on each other. Montenegro joins in too. The war is now in place.