The Notes App is now gone from my Macbook Air. How do I get it back?
The easiest thing to do might be to check in Lauchpad (the silver rocket in the dock or in /Applications ) to search for notes.

If it's still there, it just got moved, so you can search in Spotlight to find it.

If it's really gone, you can restore it using Time Machine or another backup. Worst case, you would have to re-install the OS but that might need some details about how you purchased Mountain Lion (whether it was an upgrade or came with the Mac for instance).