Adding a custom header in a theme?

I’m having trouble figuring out how to add a custom header in a theme I’m trying to create – searching around the site brought me this page, but I’m not entirely sure where to put the line: “add_theme_support( ‘custom-header’ );” because when I put it in at the end of functions.php, I get a “Fatal error: Call to undefined function add_theme_support()” and I don’t really know how to resolve it outside of removing the code entirely.

Could anyone have a suggestion as to what I’m doing wrong? (Do I need to include the functions.php code? Because it’s rather long and I don’t want to take up unnecessary space”