This is really the most basic of questions when it comes to Joomla CMS. You should consider doing a few tutorials and ‘getting started’ guides to get yourself accustomed to how joomla works.
If you go to your admin backend and then go to Extensions>Module Manager you can see all your modules. You will see one here called ‘Main menu’ with position ‘left’.
Click on this and you will see ‘Menu assignment’ with 3 options, All, various, and none. If you select ALL then your main menu will appear on all pages within your site. If you select various you can select which pages you want the menu to appear on. Highlight multiple pages by holding CTRL and clicking on pages within the list.
