David Copperfield is a Bildungsroman, which indicates a development and some kind of learning. What does David learn?
A bildungsroman can actually be called a novel of education. It depicts the struggles and challenges that a protagonist faces and how they develop and mature as a result of those challenges until they reach a stage where they are happy with themselves and have found their niche in society at large. David Copperfield is an excellent example of a bildungsroman, as we follow David from his birth right up until he reaches a place of stability in himself and in society, but only after he has learnt some very hard lessons about life and has been changed as a result.
One of the biggest lessons that David learns is not to be taken in by appearances and to be a more shrewd judge of character. This of course is something that he learns through his friendship with Steerforth. What is interesting about the narrative of this tale is that it is first person retrospective, which means that although David tells his own tale, often he allows his maturer, adult self to look back at what happens to him and his experiences and to comment on them, indicating his own naivety in his youth. Often therefore the narrative points to the young and impressionable David Copperfield who misinterprets certain actions and then also the older and wiser David Copperfield who ironically comments upon how clueless he was. Consider the following quote describing how Steerforth charmed the Peggottys:
If anyone had told me, then, that all this was a brilliant game, played for the excitement of the moment . . . in the thoughtless love of superiority, in a mere wasteful careless course of winning what was worthless to him, and next minute thrown away . . . I wonder in what manner of receiving it my indignation would have found a vent!
Note how here we see the older David commenting on how totally he was taken in by Steerforth. Even though now David recognises the charm offensive by Steerforth for what it is, David also indicates that he would have been incredibly angry at that point in the story if anyone had questioned the motives of his friend and suggested such a thing. By the end of the novel, however, with the ensuing tragedy that befalls the Peggottys and Steerforth's role in it, David has certainly learnt his lesson and he will not be taken in by such characters so easily again.
This is one clear way in which David Copperfield's character changes and develops. Hopefully you will be able to use this as an example to find some more. Good luck!