Wickham, in social settings, and on the surface, is incredibly charming, talkative, polite, courteous, amiable, agreeable and pleasant. Darcy, on the other hand, comes off as haughty, quiet, bored, rude and a bit snobby. Wickham, as a character, is fickle, money-grubbing, insincere and manipulative. He is materialistic and filled with vices and weaknesses that he constantly succombs to. He is willing to use people to get what he wants for his own life, and uses his charm to manipulate others. Darcy on the other hand, is sincere, loyal, kind, thoughtful, and not focused on money at all. He is a good man, who does not have bad habits. He is a good brother and a true and loyal friend. He lives by moral codes, wheras Wickham does not; Wickham's only moral code is to serve himself.
Once you get to know Darcy, you realize that he is not snobby at all, and very friendly and kind. His weakness is that he is shy and awkward in large groups; he admits as much to Elizabeth. Once you get to know Wickham, you realize that his charm is his only recommendable trait; he is instead a disloyal disappointment to those that care for him.
I hope that those thoughts help to get you started; good luck!
In Ch.15 as soon as Wickham makes his appearance for the first time in the novel, he runs into Darcy with dramatic results:
Both changed colour, one looked white, the other red. Mr. Wickham after a few moments, touched his hat -- a salutation which Mr.Darcy just deigned to return. What could be the meaning of it? -- It was impossible to imagine; it was impossible not to long to know.
Jane Austen thus succeeds in creating the necessary suspense to make her readers curious about these two important characters. They don't have to wait too long, because Elizabeth who is prejudiced against Darcy is only too willing to listen to Wickham tarnishing the character of Darcy in Ch. 16:
Whatever he [Wickhan] said, was said well; and whatever he did, done gracefully. Elizabeth went away with her head full of him. She could think of nothing but of Mr.Wickham and of what he had told her, all the way home.
Wickham is a handsome, charming smooth talking villain in the militia who takes advantage of Elizabeth's hatred for Darcy and almost succeeds in destroying the reputation of the Bennet family.
Darcy on the contrary, who although is Wickham's social superior, is not as handsome as Wickham and is always tongue tied and insists on following all the formal rules of polite social behavior consequently he is condemned as being very proud and arrogant:
he [Darcy] was looked at with great admiration for about half the evening, till his manners gave a disgust which turned the tide of his popularity; for he was discovered to be proud to be above his company, and above being pleased; and not all his large estate in Derbyshire could then save him from having a most forbidding, disagreeable countenance, and being unworthy to be compared with his friend. [Ch.3].
It is only in Ch.35 when Darcy whose proposal has been rejected by Elizabeth explains the villainy of Wickham that Elizabeth realizes who is who and begins to fall in love with Darcy. In the next chapter Elizabeth exclaims:
Pleased with the preference of one, and offended by the neglect of the other, on the very beginning of our acquaintance, I have courted prepossession and ignorance, and driven reason away, where either were concerned. Till this moment, I never knew myself.''
After this Elizabeth begins to appreciate the genuinely superior character of the hero Darcy and soon they are united in wedlock.