Of course, Hello, is Ni Hao. but the rest of what comes next is based on your relationship with the person. I don't think they ask Ni hao ma of strangers like we do with english. After an introduction, they might say, hen gao xing renshi ni (happy to meet you). There is the question of "ni chi le ma?" have you eatened? or Ni qu na er? where are you going if you know the person ... But if you are just meeting, and you can say Hello, they will probably start asking you questions before you can think to ask them questions.

Well, in fact, we don't really say that often to everyday friend, like classmates and colleagues. We would just say "hey"(嘿hei). I'd say "ni hao" to my customers or some strangers...

Just Ni hao!

They mean the same. But we say "nihao" more than "ni hao ma?" It will be a little strange when you say 'ni hao ma'. During the young people, we just say :" Hi! "

:) Ni hao，we use it to a stranger or a nice-to-meet-you occasion, which means we seldom use it at all. Of course, when we ask sb. for help, we say “Ni hao, 请问。。。。。。“” we never greet each other with "How are you? / what's up etc." [if we haven't seen each other for a long time, we may say "最近过得怎么样啊/去哪了都，这么久不见etc"] Just a "hi", and go straight to the point. :) But if we talk to a foreigner, we may say How are you.