You must use "tú" when addressing people you are familiar with, such as friends, peers, equals... and when ordering something in a store, for example And you should use "usted" when it comes to someone who is well older than you, maybe an elderly man or woman you don´t know. Here in Spain we address our teachers as "tú" instead of "usted" and pretty much anyone other than elderly people And remember "tú" goes with accent. Otherwise it would be "tu" which means "your".

Hi, in South America the use of "usted" is more common than in Spain, so you might hear a lot of it if you are watching South American TV.

In Spanish we use (usted) as a respect for older people or people with more Hierarchy. You must say ud to your teacher, grand parent, your boss etc. We use tú to refer o talk to your friend ,sibling, partner . Two words in Spanish(Usted, tú) for one word in English (you).