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