Someone's tried it before: http://www.nutritionask.com/
With a bunch of widgets, changes in the design. And now you can sort by upvotes too so it's probably much more possible to do a Q&A style site than it was a month ago when you first asked
I'm new to nodeBB. the new topic button doesn't do anything. do i need to add a plugin for it to work ?
You might need to activate one of the composers. The default composer should be activated, but perhaps it got deactivated by you? Give that a check.