Ender's Game. A great intro to sci-fi. It takes place in the future, after two alien attacks nearly wiped out the earth. The human race was saved by a brilliant commander's battle tactics. Ender Wiggin is recruited into Battle School, which was established after the alien attack. There, they find future brilliant commanders from the most genius kids on the earth. It's been decades since the alien attack, and a third attack is due to come soon, and this one will be the big one. Ender is just a kid, a genius among geniuses, with the fate of the world on his shoulders.
http://www.amazon.com/Enders-Game-Ender-Book-1/dp/0812550706