In the first chapter, Ralph and Piggy find the conch and use it to summon the other boys. Ralph is already establishing himself as a leader and an efficient organizer. When Jack shows up, he acts superior and asserts his own leadership. Although Jack does act like he is the obvious choice for chief, the boys sense something about Ralph that make him seem like an able leader as well:
But there was a stillness about Ralph as he sat that marked him out; there was his size and attractive appearance; and most obscurely, yet most powerfully, there was the conch. The being that had blown that, had sat waiting for them on the platform with the delicate thing balanced on his knees, was set apart.
The conch established Ralph as the organizer and therefore the leader. When they elect Ralph as chief, Ralph tries to placate Jack by offering him the choir. Ralph has made this first gesture to avoid a power struggle between he and Jack. Although he appreciates this, Jack is not as thoughtful as Ralph and the power struggle will inevitably emerge.
In Chapter 2, when Ralph calls another assembly with the conch, he says that everyone can speak but he must be holding the conch. Jack is interested in having rules but he is more excited about what they might do if someone breaks the rules. Jack is clearly establishing himself as someone who wants to be leader because of the power. Ralph wants to lead to get things accomplished. In addition to addressing their daily needs, Ralph assures them that they will be rescued and he gains their respect.
The assembly was lifted toward safety by his words. They liked and now respected him. Spontaneously they began to clap and presently the platform was loud with applause. Ralph flushed, looking sideways at Piggy's open admiration, and then the other way at Jack who was smirking and showing that he too knew how to clap.