So Bosnia and Serbia were being run by the government of Austria-Hungary and the Serbians were like "dude, I totally hate Austria" and "Yeah, dude, same here, man."
So a bunch of young rebel Serbians were like "let's make this cool secret group that meets in bakeries, and we'll be called the Black Hand and take down Austria!" and other young people were like "HECK YES!" but they were very secretive, even with all this celebrating of their genius.
So this one guy, Gavrilo Princep, and his cronies started planning the assassination of the Archduke of Austria. They went into parks and practiced shooting, but all his friends were really crap at it, so when it came time to kill Franz Ferdinand all his friends just had some lame grenades to fire and he had a gun.
Also, just in case they were caught, they were like "let's all have this cyanide with us, so no one can ever capture us. We'll be dead! HAHAHAHAHAHA."
Actually, it turned out that the cyanide was too old. So one guy tried to throw his grenade at the archduke and missed. He downed his cyanide and jumped into a river.
This would have been, like, an awesome plan, BUT THE RIVER WAS A FOOT DEEP. And because the cyanide was way old, it just made him really sick. So he was caught. The same sort of thing happened to another one.
The first time Gavrilo saw Franz Ferdinand, he didn't shoot him because the timing was off. Instead, he went and got a sandwich.
Incidentally, Ferdinand's driver was really confused about where he was going, and made a wrong turn.
This turn brought them in front of the sandwich shop where Gavrilo was standing outside, eating his Serbian sandwich.
Gavrilo was like "screw my sandwich, IT'S THE ARCHDUKE!" and shot. This shot killed the archduke and his wife.

After this, everyone blamed Serbia for the deaths. Russia was on Serbia's side, though, and Russia was like "Yo, Austria, if you try to do anything to Serbia, we're gonna take you down!"
And then Austria was like "we ain't afraid of you, fool!" and, like, glared and did that 'punch your own hand' thing and got all up in Russia's face.
And then Russia was like "We mean it, fool!" and started moving in on Austria.
At this time, all these alliances had been made all over Europe and so when Russia started moving in on Austria, they reached the German border and Germany was like "Whoa, man, whatchoo think you're doing?"
And Russia was like "Um, taking Austria down..."
And Germany was like "Oh yeah? We're gonna take you down if you don't stay outta our country!"
And so started WWI.

Basically, this is seriously what happened.