It sounds like "Nee How" and that means goodness to you. Just say the same thing back "Nee How." If you hear a "Ma" at the end of "Nee How" that means they are asking you a question but it is like English when we say "Hello, how are you?" we really don't care how they feel it is just something we say.

If someone greets to you , they may smile to you or nod their head or say 'hello' . You just do the same with he/she.