Integrating with custom theme

Hello.

We had a developer build a short custom theme for us. He is long gone but we are now trying to integrate Ninja Forms to this simple custom theme.

We are getting this error in the source code on the front end:
Notice: JavaScript is required for this content

We have both header() and footer() added to this template/page:
get_header();
get_footer();

Can someone please tell me how to add and which JS scripts to add to this template so that the form displays on the front end..??

I will have to add some JS and CSS files to the header for this to work I am presuming…??

Thanks so much in advance.