Here is a web page I found. There are two example codes to illustrate two ways to create the menu bar with items.
That web page is written in Chinese, and if you don't read Chinese, here is another web page in English. I believe the English one is the original one and it contains more details in explanation.
The new version tutorial of the Gtk+ menu widget can also been found on the GNOME library website:
You have to go through the following pages to complete that section.