To get all prepped, he commissions a souped-up truck with a little house on the back that he can live in when he isn't crashing at hotels. He calls the truck "Rocinante" after Don Quixote's horse—clever, huh? When he's all set (and after a small run-in with a hurricane just before he was supposed to leave), he and Charley (his French poodle) hit the road.
He starts out by driving over into Connecticut from his home in Long Island (with some assists from ferries, natch) and then heads north into New England. Along the way, he meets a pretty colorful group of characters and learns about their ways of life and their perspectives on the country and its politics. Also, he kind of takes the temperature of regional "temperaments" along the way.
Then he comes back down out of New England and heads west, crossing through New York. He tries to cut through Canada, but he gets into a kerfuffle at the border because Charley doesn't have his proof of rabies vaccination, so he has to turn around. Steinbeck then passes through the Midwest, continuing to offer his reflections and thoughts about the people and places he encounters along the way.