Here we go, long story begins: Many serbs believed that Sarajevo belonged to Serbia, but it belonged to Austria-Hungary. When Archduke Franz Ferdinand (Leader of Austria-Hungary) came to visit Sarajevo, a Black Hand member (The Black Hand was a group of Serbs that wanted independence for Sarajevo) threw a grenade at his car, but it missed, killing innocents. Later, the Archduke's driver took a wrong turn, and it passed Gavrilo Princip, another Black Hand member, getting a sandwich. Princip rushed the car, shooting with his pistol, and killing the Archduke and his wife, Sophie. This caused Austria Hungary to declare war on Serbia. Russia declared war on Austria Hungary to protect their fellow slavic nation Serbia. Germany then declared war on Russia. France (who made a secret alliance with Russia) then declared war on Germany. Realizing that Germany had to deal with France first, Germany invaded Belgium to get to France, but Belgium was allied with Britain, causing Britain to declare war on Germany, and then World War I began.
I hope this helps!