I hope the title is right but I keep seeing this ending to words in my Japanese books. Has this been covered in any lesson? What is the best way to learn this ending?
I don't know where, but it's bound to have been hit on multiple times in the lessons. なりました is the past, polite, affirmative of the verb なる which means "to become."
also, like Jasonさん said, this verb is so common that it's been used in a ton of lessons. so i think it's been treated as vocabulary rather than a specific grammar point, aside from Beginner Lessons 111 and 112.