Main menu navigation links and new pages

Templates can all have unique code, and your template may or may not support Custom Menus. I would find out if you can add custom menus through your Appearance > Menus tab in the admin area. If you can’t, all new pages you add from the Pages tab will most likely end up in that main menu by default.

If you can add custom menus, then you should look up the function you are referencing here to get a good understanding of how to place your code.

Also check to see if you have the option to set the menu which will be your main menu in the Menus tab.