thumbnail images

so where exactly am i posting

add_theme_support( ‘post-thumbnails’ );

I tried this once already and it screwed up my site i had to reinstall everything. is it the first line of function.php. I’m following this https://markjaquith.wordpress.com/2009/12/23/new-in-wordpress-2-9-post-thumbnail-images/, but i’m still not sure exactly where the bit of code goes.

thanks