This is the first book in the Song of the Lioness quartet.
The back of the books says:
Alanna of Trebond is no ordinary girl- her dream is to become a knight. So she disguises herself as a boy and begins training at the palace of Kind Roald. Alanna quickly finds out that the road to knighthood is not an easy one, but her skills and stubbornness help her become friends with Prince Jonathn and his followers. At the same time, Alanna makes an enemy of the prince's uncle, the overly charming Duck Roger.
Here begins Alanna's first adventure- one that will lead not only to the fulfillment of her dreams but to a magical destiny that will make her a legend.