In my experience, not many people use "Yo" to greet someone. If people use it, it is usually in a joking way, or only with their very close friends. I would not use this greeting with coworkers, bosses, teachers, parents... anyone who is not very close to me. I might use this greeting with my sister or boyfriend.

"Yo" is slang for "hi" or "hey". So, it can be used as a greeting or a way to get someone's attention, as mentioned by Matthew. You can use it with friends or peers that you can be casual with but, don't use it too much or you might come off as unsophisticated.

This morning is usually considered as a slang or very informal language. So I highly recommend that you use it with caution. It has two different usages: 1. To used to say hello to someone or 2. To get their attention