I am number Four by Pittacus Lore.
John Smith, an alien from the distant planet Lorien, has been on the run for 10 years. Along with his Cepan, Henri, they have fled from town to town throughout America, hiding from the Mogadorians who destroyed their home planet in order to rob it of its resources.
John is one of nine children sent to Earth, along with their guardians, in order to blend in, survive, and eventually repopulate their people. A protective charm prevents them from being killed out of order, and hopefully this will allow them enough time to develop their extraordinary skills.
As the book begins, a third scar burns a white-hot ring around his ankle and John realizes another one has been killed. He is vulnerable to the Mogadorians, if they should find him and attempt to kill him. He and Henri flee for Paradise, Ohio and must lay low, hoping John will develop his Legacies and be strong enough to face the Mogadorians should they attack. John manages to make a friend, the alien conspiracy theorist Sam, and falls in love with Sarah Hart.
For the first time, he develops relationships with people and as strange things begin to happen, he finds it difficult to stay hidden behind his normal, American teenager facade.