Add a slider on homepage after menu bar

It looks like you use a child theme of lawyeria-lite. Assuming the header.php template has not been too heavily altered, it would go there, below the call to wp_nav_menu() and it’s associated containers. Wrap you slider code in a if ( is_home()): conditional so it is only applied to the home page.

If your theme has no header.php file, copy it from the parent theme folder.

You could always take a look at the revolution slider plugin, i purchased the pro version but its completely worth it.