Ni hao is the first phrase of Mandarin that anyone should learn. It means literally "You good", or "I hope you are doing well", (somewhat like Spanish Que Pasa) but actually is just the standard way of saying hello in Chinese.
There could also be nin hao (more formal), or nimen hao (plural), but the basic concept of "ni hao" is really all you need to know if you want to say hello in Chinese, which is the first step you need to take.
There is another phrase Ni Hao Ma , which adds the questioning participle "ma", and turns it into a question "How are you doing?" , and which, unlike a simple "ni hao", actually demands a response, usually "hen hao" , (very well).
Also, "ni hao" should be inflected, but that is true of almost all Mandarin phrases.