Ender Wiggin failed the test of becoming a military genius at 6 years old. So they had to remove the monitor, from his neck and return him to his family where he is the third child
Colonel Graff goes to Ender's house to give him a choice of becoming a hero and saving the people from the buggers, or to stay with his family where he isn't wanted. Ender chooses to go.
Ender is taken to Batlle School with a bunch of other children who qualified. Ender has made a bunch of enemies though, so he has to fix his problems.
Most kids now hate Ender. Ender sits by himself at lunch, but soon that won't matter, the generals have bigger plans for Ender, so one day they promote him from the launchies to Salamander Army.
Ender has now been promoted to Rat Army, then platoon leader in Phoenix Army, Commander of Dragon Army, finally Ender is transferred to Command School. Ender now has control over an entire fleet, and goes into battle against the buggers.
Although Ender does win multiple battles and is the hero, in the end, he sees that the buggers never wanted to fight, and Ender makes it his mission to find the buggers a new home.