All you have to do is set the game type to 'King of the Hill'. If you are joining a game, you don't even have to do anything special, just join and play. The first player to make a kill becomes king, which is supposedly a good thing. He receives one frag for the kill. The king produces double the damage of a normal player, making it hard to kill him.
Surprise attacks are most effective against a king.
He is awarded five frags for the kill. He then takes the kings place, as king. Most players get a short chance at giving out 2x damage, as the king, before killed. Whoever has the most frags wins. A new map is usually loaded with the same settings and the fun begins all over.