This question seems to indicate that Ralph has a single act of rebellion against civilization, and I do not agree with that. I think Ralph does a fair number of things that rebel against civilization. While I don't think that Ralph ever becomes as savage as Jack, I do think that he feel stifled by adults and the rules that govern him in civilized society. I think the first rebellious act that he does can be found in the first few pages of the book: he and Piggy have been discussing their situation, and Ralph comes to the beach. Something about it overpowers any of his thoughts of societal impropriety, and he strips completely naked and runs into the water.
He jumped down from the terrace. The sand was thick over his black shoes and the heat hit him. He became conscious of the weight of clothes, kicked his shoes off fiercely and ripped off each stocking with its elastic garter in a single movement. Then he leapt back on the terrace, pulled off his shirt, and stood there among the skull-like coconuts with green shadows from the palms and the forest sliding over his skin. He undid the snake-clasp of his belt, lugged off his shorts and pants, and stood there naked, looking at the dazzling beach and the water.
Notice how the island has already made him aware of the weight of his clothes. He is tied down and burdened by that weight and strangled by the elastic garter and belt. His first rebellious act is casting off all of the clothes that a civilized society would make him wear.