There are a couple of ways of answering this question.
You could go through some of the major characters and think about the way the hard times are reflected through their actions and behaviour.
For Example
Curley’s Wife wants to escape, she thought marrying Curley was a form of escape. In fact it just turned out to be a different trap. All the other men treat her badly, they are extremely misogynistic and actually you could say that men just use women in the novel for sex. Women are attacked, killed or used as prostitutes. What does this reveal about American’s society to women at this time. Does this excuse her behaviour towards Crooks? Does Steinbeck ever describe her in an attractive way, do we ever feel sorry for her?
And then perhaps think about some other characters. Crooks and race, Candy, disability and old age, the obvious parallel with this dog.
You could look at a more thematic approach
Discrimination
Situation of the working man: Forced to travel around the country, no hope prospect for the future, split up from friends and family, isolated, mistreated by those that have power e.g. the bus driver and Curley, only escape is into drink and visiting prostitues, acts of petty cruelty over those with less power than them
Acts of violence and cruelty
Despair and disappointment
Or a chronological approach, so you analyse the events of the novel from the very beginning and work you way through.
What do you think?