Since you only have one hundred words for your summary, you will want to focus on just the most important aspects of the chapter. Just give them a brief mention and move on to the next. Chapter 1 includes a mixture of exposition and foreshadowing, all of which will become more significant as the story progresses. Since many important details are introduced at the beginning of the book that will come into play later, it is important to include these.
Chapter 1 introduces us to the two central characters of the story, Lennie and George, and provides a little bit of their background. You should mention their physical descriptions as this has significance later on. George makes it a point to remind Lennie that they are heading north to look for work so that they can someday buy their own ranch. George also reminds Lennie of his habit to pet things and how it gets them both into trouble. You can include the parent-child dynamic between the two men as well. You might also want to mention that George designates this spot by the river as the place where Lennie should come to if there is trouble, which is clear foreshadowing.