Get comments under posts

Hi.
I’ve tried so many things, but it always ends in syntax/function error etc.

I just want to have a comment-field under each post.
I know i must set the code <?php comments_template(); ?> in single.php
But what about functions.php, comment-template.php or comments.php?

Please explain step by step.
It’s for my wallpapersite

Thanks in advance.