What kind of diction dominates the first chapter in the text of Of Mice and Men? I don't understand how one can describe the diction of a text.

Diction is defined as the word choices, writing style, and techniques the author uses with narration and dialogue. In terms of narration, Steinbeck uses a limited third-person narrator. The narrator sticks to describing the setting, the landscape, and the outward appearances of George and Lennie. Steinbeck allows these surface descriptions to supplement the dialogue. The novel (novella) is therefore like a play in that it is composed mostly of setting and dialogue. 

In terms of...

Diction is defined as the word choices, writing style, and techniques the author uses with narration and dialogue. In terms of narration, Steinbeck uses a limited third-person narrator. The narrator sticks to describing the setting, the landscape, and the outward appearances of George and Lennie. Steinbeck allows these surface descriptions to supplement the dialogue. The novel (novella) is therefore like a play in that it is composed mostly of setting and dialogue. 

In terms of the diction of the dialogue, Steinbeck has George and Lennie use a lot of slang and idiomatic language. This is to make the dialogue realistic. The story falls into the Realist and/or Naturalist categories because it attempts to paint a realistic picture of the lives of itinerant ranchers of this historical period and because it focuses on how the characters are at the mercy of their environment. 

The diction of the dialogue is plain language: how people would have spoken in this cultural environment and period in history. It is what these men would have sounded like at that time. Steinbeck does this to give the dialogue sincerity and a sense of being genuine. So, the diction really breaks down into this limited third-person narration and the Realist, genuine dialogue.