The beach by the lagoon is "home." It's also where the Naval officer finds the boys at the end. The platform is where meetings take place--"the office." The mountain is where fire happens--the first huge fire that kills at least one boy, and the rescue fire. The mountain is also where the pilot lands, and the boys think it is the Beast. The mountain is also where Piggy's glasses are broken by Jack. Castle Rock is where Jack runs his tribe. This is also the source of the rock that kills Piggy. The neck (narrow passage-get it?) at the end of the island, just before Castle rock, is where Ralph and Jack fight, and the conch is broken and Piggy killed. Just inside that point is the tangle of creepers where Ralph hides and where the boys start the last big fire. The beach near there is where the boys dance and chant and kill Simon. Simon's hideout is where Ralph hides later. The site of the sow's kill is where the Lord of the Flies is mounted. Simon has his "episode" there, and Ralph takes the stick for a spear.
I hope these ideas will help you get started on your essay.