What/who creates the conflict in the story?

The government of Panem has basically created the central conflict(s) in The Hunger Games. The government has, for seventy-four years, held the Hunger Games as a reminder to the people of Panem that they cannot and should not rebel against the government. The games force a boy and a girl, drawn randomly once each year in "The Reaping," from each of the 12 districts of Panem to compete in a fight to the death. This competition is also obligatory viewing for everyone in Panem. The people in the Capitol enjoy it: they place bets, cheer for their favorites, and are completely absorbed in the events leading up to and during the games. 
Katniss, the protagonist and heroine, is faced with a number of conflicts throughout The Hunger Games. She must choose to rebel against the government more than once, including before the games, when she hunts illegally in the woods to support her family. Once her sister Prim is chosen in the Reaping, she faces a decision and ends up instinctively volunteering to go in Prim's place. Once she is training, she encounters conflicts with her mentor and the boy from District 12, Peeta, as well as the other tributes. Obviously, once the games begin, her conflicts are between herself and the other tributes whom she must kill to survive; she also faces internal conflicts, though, in determining how to "play" the games and when and if she should kill her competitors. All of Katniss's conflicts, though, are a result of the government's totalitarian rule, especially their installation of the Hunger Games as a required event each year.