Jack’s “I Am” poem would focus on demonstrating strength and power.
An “I Am” poem is usually written about oneself. There are different methods, but in general it opens with “I Am” and a description of two characteristics describing the person.
Jack would want to focus on his personal strength and power. He would play up any positive or intimidating things about himself, and play down any less-than-desirable ones. He would in general want to come across as fierce and dominant. His characteristics might be words like “strong” and “brave.”
The description of Jack Merridew is not very flattering.
[He] was tall, thin, and bony; and his hair was red beneath the black cap. His face was crumpled and freckled, and ugly without silliness. Out of this face stared two light blue eyes, frustrated now, and turning, or ready to turn, to anger. (ch1)
You will want to fill the other lines based on Jack’s personality. For example, Jack is arrogant. In the second line, he might “wonder” why anyone would contradict him. He might wonder why the boys did not choose him leader. He might “hear” the call of the beast and “see” its blood coursing through his hands.