How does Steinbeck present George's power over Lennie in the opening scene of the vovella?

Your question cites the opening scene. Gradesaver's analysos should help you formulate and answer.

In this scene George and Lennie are hiding by the river. Lennie can surmise they're in trouble by gaging George's actions, but he doesn't know what he did wrong. Lennie’s vision of nature is hardly realistic; he thinks of nature as full of fluffy and cute playthings. He has no notion of the darkness in the natural world, the competition and the cruelty. He wouldn’t have the faintest notion how to feed himself without George. In this too the men balance each other: George sees the world through suspicious eyes. He sees only the darkness where Lennie sees only the light. George may complain about how burdensome it is to care for Lennie, but this complaint seems to ring hollow: in truth, George needs Lennie’s innocence as much as Lennie needs George’s experience. They complement each other, complete each other. Together, they are more than the solitary and miserable nobodies making their migrant wages during the Depression. Together, they have hope and solidarity.

George’s complaint – “Life would be so easy without Lennie” – and Lennie’s counter-complaint – “I could just live in a cave and leave George alone” – are not really sincere. They are staged, hollow threats, like the threats of parents and children (“I’ll pull this car over right now, mister!”). Similarly, George’s story about how “things are going to be,” with rabbits and a vegetable garden and the fat of the land, also has a formulaic quality, like a child’s bedtime story. Children (like Lennie) love to hear the same tale repeated countless times; even when they have the story memorized, they love to talk along, anticipating the major turns in the story and correcting their parents if they leave out any details. “The rabbits” is Lennie’s bedtime story, and while George isn’t exactly a parent to Lennie, he is nevertheless parental. George is Lennie’s guardian – and in guarding Lennie, George is in effect guarding innocence itself.