"Ni Hao" (你好)
It's simply saying "Hello" back to them.
However, you can also add a little more:
* "Ni Hao, (name)" (你好, (名字)) - If you know their name, add it for a more personal touch.
* "Ni Hao ma?" (你好吗?) - "How are you?"
* "Wo hen hao, Xiexie." (我很好, 谢谢) - "I'm good, thank you." (You can use this after someone asks you "Ni Hao ma?")
Remember, the most important thing is to be friendly and polite! 😊