This question is a little vague, so I will interpret it one way, hopefully the way you need:
Evaluating a piece of literature requires, first, placing it in literary history -- period, genre, author's development, etc. Then you should make a preliminary statement about its large topic -- love, death, social change, etc. -- some large category based on your first reading. Next, focus closer -- image clusters, tone, rhythms, etc. Then start observing details -- how the author uses language choices, the order of information given the reader, etc. And so on --the metaphor is focusing like a camera -- closer and closer -- The more attention to detail, the more "valuable." Now, the term "evaluate" implies stating the piece's "worth -- its importance, its essential truths, its depth. We could say, for example, that John Milton's Divine Comedy has more "value" than a poem by Edgar Guest. Hope this helps.