how did the boys come to be on this island? who is Merridew, and what isour first impression of him?

During an unnamed time of war, a plane carrying a group of British schoolboys is shot down over the Pacific.

http://www.gradesaver.com/lord-of-the-flies/study-guide/short-summary/

Jack Merridew

The leader of a boys' choir, Jack exemplifies militarism as it borders on authoritarianism. He is cruel and sadistic, preoccupied with hunting and killing pigs. His sadism intensifies throughout the novel, and he eventually turns cruelly on the other boys. Jack feigns an interest in the rules of order established on the island, but only if they allow him to inflict punishment. Jack represents anarchy. His rejection of Ralph's imposed order--and the bloody results of this act--indicate the danger inherent in an anarchic system based only on self-interest.

http://www.gradesaver.com/lord-of-the-flies/study-guide/character-list/