Jack's overall appearance and personality make him the powerful leader that he is. He is described as taller than the other boys. That alone gives him an advantage. Then his outgoing personality and the fact that he is the choir leader gives him position in their social circles. Another aspect of his personality is his arrogance. He feels that he is better than the others, especially Piggy and the Littluns. He is vocal in how he feels and even volunteers himself to be the chief. His moments of violence even in the beginning (when he steps on Piggy's specs) show his power.
Golding creates this monster of a character for us to represent all that is savage and potentially evil in all of humankind. He wants us to see how far kids would actually go and he represents the potential "evil doings" in all of us. Jack is the extreme, as is Roger, but with Jack's leadership, Golding shows us how controlling one person can be, and how easy it is for a society to completely lose control by following the wrong person.