An introduction should perform three functions:
- draw in the reader
- transition to the thesis
- use a thesis to explain the purpose of the paper.
If writing about Crooks, to hook the audience, you might consider some of the racial injustices in the world. A good hook or lead (that 1st sentence or 2) might shock the reader with a statistic, tell an engaging story, or relive a part of the text that most demonstrates racism, prejudice or injustice. Another way to begin is to just sum up some of the relevant scenarios from the book: I teach my students to T.A.G. that first sentence sometimes. Use the Title, Author and Genre to get into what the book is about.
Use a middle part of your intro to connect topically to your thesis.
I don't know what you have chosen your body paragraphs to be about, but I often encourage students in 9th grade to write a thesis that contains these elements: topic + strong verb + BP1 + BP2 + BP3. The order is not important, but this is a good simple styled thesis.
Yours might look like this: "Steinbeck portrays Crooks through imagery, characterization, and dialogue in order to develop his character."
Hope that helps!