The only themes we can use on free hosted WordPress.com blogs like your blog are the themes found here http://wordpress.com/themes/
Please read this section of the support docs http://en.support.wordpress.com/themes/premium-themes/
There is no FTP access to free hosted WordPress.com blogs and we cannot upload any third party themes, create child themes or our own themes or edit templates.
There is no upgrade you can purchase that changes that. Please read > http://support.wordpress.com/com-vs-org/
WordPress.org offers free software that you can install on a web server. You can upload and install themes and plugins, run ads, and edit the database. Learn about the differences here. http://support.wordpress.com/com-vs-org/
Also note that premium themes and all upgrades provided through WordPress.COM can only be used on free hosted WordPress>COM blogs and no others.