The most important pre-requisite to literary criticism is the ability to read like a literary critic. You must read not only for content and understanding of the subject, but you must also be careful to read for any and all literary devices, as well as trying to understand the reason and motivation behind every line. This is a difficult habit to get into, but it is helpful to take notes while you read.
In other words, a careful reader does not just read a text for the 'story'. Rather than just being interested in what happens or what is being described by a literary work, a critical reader is interested in how the writer has used language, form and style in order to convey meaning and affect the reader's response to the subject matter. A critical reader pays attention to patterns in texts and to how these establish tone, register, atmosphere and mood.