I'm assuming you need a paragraph contrasting the two characters, right? Also, I'm not sure what you mean by the "beginning" of the novel, so I'll try to keep the references from early on in the novel.
First, I would recommend starting with a generalized observation of their personalities, such as "Amir appears to be a coward, unlike his servant and 'friend', Amir". You can then provide quotes from the text to support these observations.
For example, when Amir and Hassan run into Assef and his friends, Amir does not want anything to do with Assef, despite Assef's insults and threats: "'Just let us go, Assef,' I said, hating the way my voice trembled. 'We're not bothering you'" (Hosseini 41). On the contrary, Hassan is the one who steps in and saves them from being beaten: "Hassan held the slingshot pointed directly at Assef's face.... 'Please leave us alone, Agha,' Hassan said in a flat tone" (Hosseini 42). Hassan was scared, but he stood up for Amir regardless of what he was feeling.
You could even borrow a passage from the end of chapter three where Baba comments how "[t]here is something missing in that boy" (Hosseini 22), referencing how Amir gets bullied in the streets and doesn't stand up for himself.
As with all cited evidence, be sure to (1) introduce the quote, (2) provide the quote, then (3) explain its significance/importance to your claim.
Other contrasting points could be how Amir is unfaithful while Hassan is loyal, or perhaps how Amir is spoiled but Hassan is thankful for everything he has. You could provide ample evidence from the text to support these as well.