what starts the actual fighting between the boys? how could ralph have prevented the fight ? or could he?

Ralph calls Jack a thief for stealing Piggy's glasses. I don't think Ralph could have avoided this, it pretty much had to happen at some point.

Jack and Ralph immediately face off. Jack commands Ralph to leave his camp, and Ralph demands that Jack return Piggy’s glasses. Jack attacks Ralph, and they fight. Ralph struggles to make Jack understand the importance of the signal fire to any hope the boys might have of ever being rescued, but Jack orders his hunters to capture Sam and Eric and tie them up. This sends Ralph into a fury, and he lunges at Jack.

Ralph and Jack fight for a second time. Piggy cries out shrilly, struggling to make himself heard over the brawl. As Piggy tries to speak, hoping to remind the group of the importance of rules and rescue, Roger shoves a massive rock down the mountainside. Ralph, who hears the rock falling, dives and dodges it. But the boulder strikes Piggy, shatters the conch shell he is holding, and knocks him off the mountainside to his death on the rocks below. Jack throws his spear at Ralph, and the other boys quickly join in. Ralph escapes into the jungle, and Roger and Jack begin to torture Sam and Eric, forcing them to submit to Jack’s authority and join his tribe.