This is a good question. There are many symbols in the book, Lord of the Flies. In light of this, let me give you a few of them.
First, the most obvious symbol in the book is the conch. When the conch is found and more importantly blown, it brings the boys together. It is a symbol of civilization. This is why it is also significant when the conch in broken. When the conch is broken (as Piggy dies), it is a sign that civility is now gone as well.
Second, the glasses of Piggy are also a symbol of technology and science. It would be good to remember that the boys use the glasses to make fire. It is also no wonder that Jack seeks to steal it.
Third, on a more abstract level, Simon is also a symbol. I would say that he is the Christ symbol as he dies, as a sacrifice. He is also the most religious boy in the group.
These symbols should get you started.