How do I pin a comment to the top of the comment list on my videos? and can it be any comment? or one of mine / not one of mine only?
It can be any comment.
Strangely, it can't be done from the creator studio or even the "community" (comments page)
You have to actually go to the video's "normal" page. Find the comment you want to pin, click on the 3 dots on the far right of the comment, select "Pin" and then confirm by clicking the blue "pin" button again. (If you already have a pinned comment, it will replace it. You can only have one pinned comment.)
It's a very useful feature for displaying important info that might not be read in the description.