Steinbeck's novel offers a message about the importance of community. People need to stick together and help each other if they want to survive and prosper. The Great Depression is the backdrop to this novel, a time when the conomy had collapsed due to the 1929 stock market crash, and millions of people were unemployed. During the 1930s, debates raged about how to solve this crisis. Steinbeck saw the answer in ordinary people coming together to organize and work on behalf of one other.
The rich, he believed, would do their best to keep ordinary people divided. The rich benefited when they kept average workers at each other's throats and competing for lower and lower wages.
An example of two ideologies, cooperation and competition, coming into conflict emerges early in the novel, in chapter two. A trucker is forbidden by his company to pick up hitchhikers. The trucker, however, reluctantly stops for a hitchhiker, though he fears he will get in trouble for it, because to help another person is to be a "good guy." That, in a nutshell, illustrates the novel's message: good guys stick together and help each other out. Steinbeck hoped people would unite in bigger way, but stresses in his novel that every act in which people lend each other a hand, no matter how small, is valuable.
Certainly, the idea of people pulling together to survive tough times is an important theme of Steinbeck's book. You could extrapolate that out that people will always have a better quality of life if they focus on the community around them, rather than on the individual.
Related to that is the theme of hope. Even though the book paints a really horrible picture of the migrant workers' lives, they are ever hopeful that it will get better. That is another way that people are able to survive the tough times. As Studs Terkel said, "hope dies last."
Last, you could make a case for the idea that Steinbeck is criticizing the class system - the powerlessness of the poor vs. the strength of the rich/dominating.