Hello Reader's! If you are new to WordPress and you have to create the menu for first time. You just have to do the steps below:-

Step1:- Open or Create a file functions.php in your theme folder and paste the code below:-

add_action( 'after_setup_theme', 'cmart_setup' );
if ( ! function_exists( 'cmart_setup' ) ):
function cmart_setup()
{
    // This theme uses post thumbnails
    add_theme_support( 'post-thumbnails' );
    register_nav_menus( array(
        'primary'       => __( 'Primary Navigation', 'ThemeName' ),
        'secondary'     => __( 'Secondary Navigation', 'ThemeName' )        
    ) );
}
endif;

Now open header.php and in place of static menu paste the code below:-

 <?php wp_nav_menu( array(
'theme_location' => 'ThemeName' ) ); ?>

Now you can see the menu option is showing on Appearance tab