Install the C/C++ plugin in NetBeans.
In NetBeans, go to Tools → Plugins.
Then in Available Plugins, in the C/C++ category, check the box for C/C++, then click Install to start the installation.
Source: Enabling C/C++/Fortran in the IDE
If no update centers are enabled in NetBeans for installing and updating plugins, then you may see a completely empty list under Available Plugins. (Less commonly, if some are enabled but they do not provide the C/C++ plugin, then you may see some plugins but not that one.)
The solution is to go to the Settings tab, look in the list of Update Centers, and check the box for NetBeans Distribution.
Then you can go back to the Available Plugins tab and the C/C++ plugin should be listed. You may have to click the Check for Newest button (though I didn't have to).