I would recommend Tamora Pierce's "The song of the Lioness" books:
The Song of the Lioness
Alanna of Trebond (later Alanna of Pirate's Swoop and Olau), is a ground-breaking female knight. Although noble-born girls traditionally go to the convent to learn how to be proper noblewomen and wives, Alanna desires to become a knight, a position only given to noble-born boys (although, as seen in the Provost's Dog books, there had historically been other lady knights). In order to achieve her dream, Alanna disguises herself as a boy under the name of Alan and goes to the palace in place of her twin brother Thom. As Thom has no interest in becoming a knight, and is far more interested in his magic, Thom goes to the convent in place of Alanna, to become a sorcerer. The twins' father is concerned only with his studies, and pays them little attention, which helps their masquerade. He is also very strongly against magic, as he believed that it should have saved his wife when she died in childbirth, so he is opposed to the idea of Thom becoming a sorcerer. Alanna has a few relationships thoughout the series, including Prince Jonathon, George Cooper and Liam Ironarm. The series follows Alanna's training and the early years of her knighthood, a timespan of ten years.
Also very good are the works from Alexander Lloys:
The Chronicles of Prydain is a five-volume series of children's fantasy novels by author Lloyd Alexander. The stories detail the adventures of a young man named Taran, who is awarded the honor of Assistant Pig-Keeper but dreams of being a grand hero, and his companions Princess Eilonwy, Fflewddur Fflam the wandering bard and king, a feral yet gentle creature called Gurgi, and a dwarf named Doli.
I devoured these books at about 12 I think...