The IOC chooses it.
There is an International Olympic Committee that chooses a city to host the upcoming games. The city that is hosting the next games, organizes it.
Cities interested in hosting the Olympic Games submit a bid to the International Olympic Committee. The bids are evaluated by the committee who then makes the choice of the host city.
London, United Kingdom is the host city of the 2012 Olympic Games.
Seoul was the first city to host the Summer Olympic Games. In 1806.
Each Country get famous sports personalities to campaign on why their country should host the Olympic games. then the Olympic committee decide.
The IOC has a bid in which cities apply and the IOC announces the wining city about seven years before the Olympics are held there.
Mexico City
Paris
Paris
London
Stockholm
None