For uploading my theme, I used the instructions from
http://wordpress.org/download/ but ended up having to call my hosting site for something really minor. Not sure if this would be the same for Joomla but hopefully it will help. FYI,
www.wordpress.com is for the free sites but
www.wordpress.org is for the self hosted sites.
For me, choosing a template was difficult as there was always something that I wanted to change. In addition, all designers are not created equal so it can be a little difficult to figure out which file to alter and where that file is (or is not). For someone like me, newly trying out my XHTML, CSS and PHP wings, unsupported, nonstandard coding got a little frustrating. WordPress; however, has more information that you can imagine so, keep
www.codex.wordpress.org in your back pocket as it appears never ending. Also,for a plain Jane idea of what the Wordpress templates/themes are based on, look over or print off the code from their default themes (Kubrick, I think, is one of them). It serves as kinda a cheatsheet for me and made it easier to understand.