Alanna The First Adventure

This book is written by Tamora Pierce.

Alanna doesn't want to be sent to become a lady, she wants to be a knight. Her brother doesn't want to be a knight, he wants to be a sorcerer. So the simple solution is - switch places.

Alanna soon learns that pretending to be a boy to become a knight is not as easy as she thought it was going to be, but she is determined to prove to herself and everyone else that she can do it.