Moving from WordPress.org hosted to wordpress.com

I have 5 WordPress sites. Four are hosted and one not. I want to have all of them on wordpress.com. How can I move the 3-4 years of content I have on my hosted site to wordpress,ocom? When I export from my ISP, I seem to get only a tar.gz file. WordPress.com does not seem to want to import such a file. I don’t see any posts on this since 2009. Is this difficult or easy? Where does one find information?