We do use ‘yo’ in a very informal way with friends and also with people who we don’t know (we don’t know their names and we want their attention).
The proper response to “what’s up” is to tell the person one of two things:
1. If they are with you at the time and looking around the area TELL THEM WHAT YOU’RE DOING. They are probably thinking something interesting is going on and want you to inform them. (Note: They may also be asking if there is a problem — listen to the tone)
2. If they are looking at you they want to know how you are. SO TELL THEM… lol!
The proper response to “How are you doing?” is
Formal: “I’m well, thank you, and yourself?”
Informal: “I’m good, and you?”
If you know some other ways to say hello or goodbye in English, leave a comment below!