Magician's Gambit (The Belgariad #3) by David Eddings

Ce'Nedra, Imperial Princess of Tolnedra, is confused. Everyone knows the tales of the Orb protecting the West from the evil god Torak are just silly legends. But here she is, forced to join a dangerous quest to recover that stolen Orb. No one believes in sorcery, but Garion's aunt and grandfather seem to be the fabled sorcerers Polgara and Belgarath, who would have to be thousands of years old.

Even young Garion is learning to do sorcery. He's just a farm boy, totally unsuitable for an Imperial Princess. Yet for some reason, she has the urge to teach him, brush back his tangled hair, and comfort him. But he is going to a strange tower in the center of all he believes evil, to face some horrible, powerful magician, and she can't be there to watch over him. She may never see him again!

Thus continues The Belgariad, an epic prophecy still unfolding.