Difference between USED TO and USED FOR - Basic English Grammar

In this video, Alisha answers 5 questions. - I would like to know the differences between the verb "to be" and "to become". For example, which one is correct? "I want to be a teacher" or "I want to become a teacher"? - What is the different between used to and used for? - What does it mean when someone says "I'm not practicing the law anymore."? I'm a little confused the word of 'practice' here. - What does "I'm blown away" mean? - What is the difference between "lot of" and "plethora of".