A theme is a distinct and recurring subject or idea contained in a story. Most stories (or novels, movies, or poems) contain multiple themes, and part of your job as a literary critic is to determine which theme you think is most significant to a literary text. What is the story really about? Here are some themes in Steinbeck's Of Mice and Men:

Companionship
Mercy
Class
Gender
The American Dream

A thesis statement is an argument about a theme in a literary work. For example, one could argue that Steinbeck's Of Mice and Men is a story about how the promise of the American Dream harms Americans more than it helps them.

Every story, novel, poem, or movie also contains at least one significant conflict, even if that conflict simply takes place within the mind of one character. While conflicts can be expressed in general terms (man vs. man, man vs. himself, man vs. nature, and man vs. society), it's best to express them in terms specific to the novel (Lennie vs. George, etc.). Try to identify the central conflict(s) in a literary text in as much detail as you can; for example, the conflict between Lennie and George is a very complicated one. Lennie is a burden to George, but he also gives George a purpose.