The easiest way to do this that doesn't vary with the type of document you want to create (application dependent) is via the terminal and the command:
touch /path/to/filename which will create a file where filename is the name of your choice in the location specified by the path.
You can open a terminal with CtrlAltT or or by searching the Activities menu. I recommend the former for those who hate wasting time.
The benefit of this approach is it works across all supported Ubuntu desktop environments and versions I've encountered and allows for increased speed by uitlizing tab completion.
The alternative is to open the application that supports the type of file you want to create, create the document and select save as. Simply launching the application will take more time than the approach I've outlined here (although you'll have to do that to edit it anyway)
The GUI doesn't appear to have the "New Document" option although you can create one. Not a great loss for CLI geeks like myself, but certainly a pain for those of us who live and breathe the GUI.