How would I go about in writing an assignment on how friendship is portrayed in Of Mice and Men and The Kite Runner?

This task can be pursued in a couple of ways.  The first
would be to develop a thesis statement about how friendship is shown in both works. 
This is going to be dependent on your own analysis and your own understanding of the
material.  Basically, it comes down to how you see friendship in both works.  Ask
yourself how George and Lennie operate as friends.  Are they "good" friends to one
another?  Do they support one another?  Where do we see instances where they are good
friends and where are there moments where they are not good friends to one another?  Is
there sacrifice between them, a necessary component of friendship?  Then, ask these same
questions of Amir and Hassan.  After this, I think being able to contrast how the
friendships of both sets of characters are different from one another will help you
immensely.  Essentially, I think you will find that both are good friends to one
another, although the concept of friendship is a bit different.  I don't think you will
get far without speaking to the idea the George and Lennie stick together until the end
when George has to shoot Lennie and that Amir and Hassan experience a distancing in
their friendship, only to be salvaged when Amir returns to "be good again."  I think
that experiencing the friendships in these questions and contexts will help out a great
deal in composing this paper.