What started world war one???

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Assasination of Austrio-Hungarian Arch-Duke Franz Ferdinand in 1914 by Serbian nationalist Gavrilo Princip. This led to Austria-Hungary invading Serbia, Russia declared war to protect Serbia, Germany to protect Austria, France to protect Russia, Britain to protect Belgium, and all of a suden Europe was engulfed in the biggest war in its history. Why this political assasination set off a chain of events leading to WWI is much more complex.
All the powers of Europe were very paranoid at this time, spending huge amounts of their national resources on defence projects, and trying to outdo their perceived competitors. France, Germany, Austria-Hungary and Russia all maintained large conscript armies of millions of men for use in the event of a war which many considered inevitable, Britain focused more on maintianing command of the seas through the power of the Royal Navy. All the powers made highly complicated military plans, to by activated in the event of a war, such as the German Schlieffen Plan, and the French Plan XVII. This presented certain problems, it meant that countries could not step back from the brink of war, because they would reach a point were to do so would massively compromise their military plans, giving the enemy a potentially decisive advantage, once the plans were set in motion, they could not be stopped as to do so would leave a country vulnerable to attack by the massive armies of their neighbours.
Another factor would be rivalry between specific nations, such as British naval and colonial rivalry with Germany. Both nations built large and powerful fleets of battleships, in fact the only reason that Germany had such a large navy during this period was specifically to counter the power of the Royal Navy. Also France was very paranoid about German invasion, having been invaded by Germans twice in the last century (1814 and 1871), and was determined not to be defeated again.
The causes of WWI are very complex, and this just barely scratches the surface, there are literally hundreds of books about this stuff.

There is no one cause. A series of events, triggered by the the assasination of Archduke Ferdinand, started the ball rolling. IMO The Schlieffen Plan was the biggest contributor. Once put in place it was impossible to stop.

what started it was the assassination of archduke ferdinand in sarajevo.what caused it is a little more complex.

The assassination of Archduke Franz Ferdinand in Serbia on June 28, 1914.

More broadly speaking, it was competition among the various European powers that led to WWI, specifically the various arms races in which many were engaged and the global imperial expansion of the late 19th and early 20th centuries. The major powers each came to think they had a leg up on another and that a show of strength and perhaps a few swift, easy victories would finally knock another key player out, but before they knew it, they ended up in a huge, bloody mess that nobody knew how to stop. It just went on and on until the major powers of Europe began running out of young men.