Alanna: The First Adventure, by Tamora Pierce

Alanna is not like most girls living in Tortall, she wants to be a warrior, not spend her days learning to be a lady in some dreary convent. Through a stroke of genius, she manages to become a page at the royal palace, training to be a knight. Disguised as a boy, here she will make good friends, and deadly enemies, as she tries to prove that a girl can do everything a boy can.
This book is the first installment in a thrilling quartet, that any fantasy-lover is sure to love!