If you intend to blog here at WordPress.com on the blog linked to your username or on any WordPress.com blog then you need to know that the only themes we can use on WordPress.com blogs are found here > Appearance > Themes
You cannot build your own theme here . There is no FTP access to free hosted WordPress.com blogs and we bloggers cannot install any plugins or upload any third party themes or software. Those are for WordPress.ORG installs which run on different software.
There is no upgrade that you can purchase from WordPress.com that provides FTP access, the ability to install plugins, third party themes and software into free hosted WordPress.com blogs.
Read this to understand the differences please > http://support.wordpress.com/com-vs-org/