The tributes are chosen each year by a random draw out of a bowl of names, and the amount of times your name is put into the bowl depends on how old you are and how many times you have volunteered to put your name in extra times for more food.
Tracker Jacker Attack / Chapter 14
The tributes are required to be dressed up by their stylists and get onto a carriage with the other tribute and ride through the heart of the capital, where they make first impressions to the thousands of potential sponsors that are waiting for their arrival.
Rue's Death / Chapter 18
Just as the Games are about to begin, the tributes will be placed onto stone pedestals where they have to wait 60 seconds to start playing. If they step off the pedestal for any reason, the landmines will activate.
Poisonous Berries scene / Chapter 25
Katniss was trapped in the tree by the Careers, and she was alerted towards a trucker jacket nest by another tribute, 12-year-old rue. She had the idea to saw the branch it was on down, making the nest fall down onto the Careers camp, killing one of them and making the rest flee the camp.
Rue was the 12-year-old girl tribute from tribute 11 who was allied with Katniss. They were only allies for about 40 pages, but she was one of the best allies of Katniss throughout the book. Rue was eventually killed by the boy from district 1 with a spear while Katniss ran after Rue's terrified screams.
Earlier in the book, there was an announcement in the arena that made it so that two tributes from the same arena could both win if they were alive at the end of the games. Both Peeta and Katniss survived, but when they did, the game makers told them that they were lying and that only one could win. In response, Katniss took out some poisonous berries and pretended to eat them to trick the game makers into letting them both win.