Question

Would you use tú or usted when speaking to: Your best friend? Your doctor?

Answer 1

tu is a less formal version of you, you're going to want to use usted when talking to anyone older than you, but its probably different in friend groups.

Answer 2

Tu (I don't have the accented u) is informal, so you'd use it for your best friend. Informal language is used for people you can speak casually to.
Usted is formal, so you'd use it for your doctor. Formal language is used for people you must respect, or can't speak casually to, like a teacher or someone older than you.