Hi, and welcome! There was a recent site upgrade, which omitted the most obvious link to the free-labels page; you can find it here:http://www.bookcrossing.com/labels
There's also a link to that page in the "how to BookCross" section under the "information" tab.
Note that, at present, only the blank labels on that page are free to all; just print them out and write each book's BCID in as you register your books. (The prenumbered labels currently require "wings" membership, but unless you want to register a lot of books at once, there's no real advantage to the prenumbered labels.)
You *can* register books without printed labels of any kind; you could just write the registration information in the book by hand. But if you prefer printed labels, the free PDF file from the link above should do the trick - and there are also a number of member-designed labels, if you'd like to try different styles:http://www.bookcrossing.com/---/358601
Hope you enjoy BookCrossing!