The most important part of an essay is to be sure you begin with a clear thesis. In considering these two characters, think about where they both begin as young cubs and then how they transform as the conflict develops. Then try to work those similarities and differences into overall categories that will direct the flow of your paper. I might begin with a thesis like this:
Although both Simba and and Nala show great leadership potential, Nala's sense of loyalty to the pride is needed to help Simba overcome his feelings of guilt.
In the first body paragraph, I would examine the characteristics which make both young cubs potential great leaders. They are independent thinkers, deciding to live adventurously—and sometimes in spite of their parents' instructions. Simba declares that he "[laughs] in the face of danger," and Nala is quick to pin Simba in their energetic games, showing her independent and adventurous spirit. They visit the elephant graveyard and question Zazu's indication that they will be married one day. Simba decides that when he's king, their betrothal will be the first rule that he will change. Neither cub has a passive personality, but one which questions the world around them, which great leaders need.
I would then look at how Simba's leadership potential falters after he experiences the tragic loss of his father. Simba is filled with guilt, and he loses sight of who he is. He is easily deceived by Scar and doesn't even look back at the pride he's left behind. Although "Hakuna Matata" is a fun philosophy to live by, it doesn't do justice to the memory of his father, and it doesn't help protect Nala and his mother, whom he has also left behind. The pain of his father's loss and his sense of guilt in that accident are too much for Simba to face, so he abandons his entire life and his rightful inheritance.
Nala, on the other hand, is constant in her leadership skills. When the Pride Land is near total devastation, it is Nala who seeks help—and finds Simba. It is Nala who plants the seed that it is Simba's duty to return. Although Nala doesn't carry the same sense of guilt that Simba faces, she is hurting over the loss of the life the pride once shared. Nala has faced struggles during the time that Simba has been gone, but she remains a leader among the pride members, and it is this sense of loyalty that ultimately saves them. Without Nala, Simba would never have come back.
In the conclusion, you could examine how the combination of the two characters ultimately saves the pride and defeats Scar. Although Simba becomes king, he couldn't have learned the truth and defeated Scar without Nala's sense of constant faithfulness to their pride.