There are a couple of ways to approach this. I think that anything here has to be prefaced with the idea of whatever your teacher is asking or whatever the task requirements are will be critical in your writing the introduction. You will need to make sure what is gained here is concurrent with what you need to do within the task requirements.
An introduction to any paper on themes of a particular work might start out with a basic exposition to the work, itself. This is not plot summary, as much as it is exploring out the basic idea of how the work reflects certain themes. This might involve the exploration of the historical background of the work and what Steinbeck, himself, felt about his work. Such an opening might enable you to talk about the themes in the work with ease. Another opening might be to talk about the general style and subject matter of Steinbeck's work. There might be some overlap between other works he has written and this one, enabling discussison of themes to happen with ease as they are a reflection of Steinbeck's own ideas. I think that the most direct way might be to simply discuss what the novella discussed, what ends up happening to the main characters, and then introducing the themes that you are going to discuss as an extension of this explanation.