Add sidebar in inner pages

This question has been answered many times already on this site.

There’s 2 simple steps:

One: Add to functions file

register_sidebar( array(
    'name'         => __( 'Your Sidebar' ),
    'id'           => 'wpsites-sidebar',
    'description'  => __( 'Your Sidebar Description.' ),
    'before_title' => '<h1>',
    'after_title'  => '</h1>',
) );

Two: Add where you want to output sidebar

<?php if ( is_active_sidebar( 'wpsites-sidebar' ) ) : ?>
    <ul class="wpsites-sidebar">
        <?php dynamic_sidebar( 'wpsites-sidebar' ); ?>
    </ul>
<?php endif; ?>

For further learning, look in the Twenty Fourteen default theme to see how its done.

register_sidebar
dynamic_sidebar