What should I say when my friends asked me " What's up " ?
If your friends ask you " What's up ?" , what do you say ?
Can I just say " Nothing happen " ?
Do you have more proper way to respond this question ?
Please comment.
Thank you.
Have a nice day.

Question

What should I say when my friends asked me " What's up " ?
If your friends ask you " What's up ?" , what do you say ?
Can I just say " Nothing happen " ?
Do you have more proper way to respond this question ?
Please comment.
Thank you. 
Have a nice day.

Answer

"Not much. What's up with you?"  If the "what's up" is used only as a greeting, this is an OK non-committal response.

Answer

If they are from the US, I believe you can just answer "what's up?' as well, and start a real conversation. From my experience, americans often don't expect an answer to "what's up" (except maybe "nothing much") nor to "how are you". True story!

Answer

You can reply not much, basically is asking you what is going on