Need help using custom fields

I have read the “https://codex.www.ads-software.com/Custom_Fields” and It says to add this line of php to the “Loop” and it looks as if my theme does not have one? sorry I am not a php coder at all.

What can i do. so that if an author uses my key and put in a value. I can eco at the bottom of the post “[via $somevariable]”