Critical thinking HELP!!?

OK SO i have been trying to understand how to critical think, i mean like my cl read two books great Gatsby, death of a salesmen and we were suppose to figure out what the authors were trying to say and other stuff similar to that. I'm having a really difficult time with this critical thinking stuff. i would really appreciate if someone tells me how to do it/what to do or recommends me books to read for which i can do the stuff what we have done in cl..thanks so much =)