Literary theory offers different approaches for studying and analyzing literature. There are many schools of literary theory (for example, Marxism, feminism, ecocriticism, postcolonial theory, etc.) and any work of literary criticism that you read may employ one or more of them. The following resources offer an introduction to literary theories and how you might apply them to your own analysis of a work of literature.
Beginning Theory by 
Literary Theory by 
Literary Theory: A Very Short Introduction by 
The Oxford Encyclopedia of Literary Theory by 