What is the plot of Ender's Game?
Orson Scott Card's sci-fi classic, Ender's Game, follows the life of Andrew "Ender" Wiggin, who is being groomed as Earth's next great military savior--though he is only six years old. The Earth is being threatened by an invasion of aliens--known as Buggers--and the military has reached into the elementary schools to train their next techno-warriors. The novel follows Ender's rise from Battle School hero to the International Fleet's headquarters on Eros. When he aces what he believes to be his final exam, Ender finds that the "game" has been real, and he has exterminated the entire race of the dreaded Buggers. A national hero at the age of 11, Ender is praised as the savior of the Earth, but his conscience is never at peace for his earlier murders at Battle School and for his termination of the Buggers. He eventually becomes the governor of one of the Buggers' former colonies, discovering a lone Bugger egg--the last of its species. Ender decides to find a new home for the egg so that the Buggers can repopulate their kind once again.
I've linked to a chapter by chapter summary so that you can see the plot for every chapter of the novel. There are several plots running through Ender's Game and it'd be a gross oversimplification to try to do it all here. Yes, there's the overarching plot of turning Ender into a brilliant commander to fight off the Buggers. However there are also the subtleties of motive and character interaction that give the plot a rich layer of meaning.
Also keep in mind that Ender's Game is the first book of a long and wonderful series and so you're in for a sweet ride.