Usually a comparison/contrast essay will be assigned on one theme that two stories share. If it is not assigned, then the student will need to determine what controlling idea both stories share; this controlling idea is a message that comes through the content of the story. For adolescent literature, this message is often something about maturing or taking responsibility; it can also be about love or the generation gap. (Whatever is the theme, the student should state this in a sentence.) This link lists common themes: 

http://homeworktips.about.com/od/englishhomework/a/samplethemes.htm

• Once the theme is decided upon, then the student needs to peruse the stories again, noting how the theme is developed in each story, jotting down ideas and sentences from the narratives. One method is to make two columns with Similarities and Differences at the top of each column. As the student rereads, he/she completes the columns, making notes on where the ideas are in the stories. When finished, the student can reread what is put down and decide which ideas can be supported with details from the stories. Or a Venn diagram can be made (There is one shown in the link below on writing a comparison/contrast essay). In the part where the circles overlap, the student can write the similarities; the outer parts are for the differences.
• Having completed the list or Venn diagram, the student should decide which points to use. When writing, it is better to discuss both stories point by point rather than writing about one, then the other story. On the rough draft, write the thesis that includes the theme the stories share.

e.g. In both "story A" and "story B," the young protagonists come to realize that there is much to be learned from older persons. (What they learn and how they learn it can be compared, contrasted)

• The student will want to start with the strongest comparisons and differences and discuss them. The student should include passages from the stories as supporting details for any points made. (Proof)
• The use of connecting words that show comparison or contrast is essential to a unified essay.
• The student ends the essay with a summation of the main points and a restatement of the theme.

Listed below are links that offer how-to's and provide examples of a compaison/contrast plan of organization.