Let me put this in terms a teenager can understand
Archduke Franz Ferdinand was in Sarajevo and Gavrilo Princop popped a cap in his ass
Austria-Hungary got pissed off and asked Germany for help who gave them a "do whatever the fuck you want" or a blank check. Austria-Hungary finally asked Serbia to stop all anti Austria-Hungary activities and let them investigate the murder themselves. Serbia said fuck no and then Austria-Hungary declared war
because Russia was an allie of Serbia it began mobilization (preparing for war) and Germany didn't like this so they said "back the fuck up" to which Russia said no so Germany declared war on Russia and for no apparent reason whatsoever (just to be assholes I suppose) declared war on France. the declaration of war by Germany on France pissed off the English so England declared war on Germany
so basically they're all fighting when one day Germany shot a torpedo and sunk the Lusitania. the United States put their foot down and decided enough is enough and they were going to enter the war.
the Germans shit their pants and I can't remember what happened next but that's how it started