How do I mark a book as ‘currently reading’ in the kindle paperwhite?

It’s one of the goodreads functions, the ‘G’ icon at the top in the menu bar should get you there, and you can do all that stuff there. I think if you’ve logged into goodreads and you start a new book it also is in the popup you get at the beginning of the book, but I’m not sure how to get to that again after you’ve already started the book.
And notice, this feature is only available for US users.