I'm sorry, but themes can't be uploaded to WordPress.com. That's a feature of self-hosted blogs through WordPress.org. Please see here for more information: http://en.support.wordpress.com/themes/adding-new-themes/
You may use any of the available themes we have under Appearance > Themes in your dashboard, and you may want to look into purchasing the Custom Design Upgrade to customize one of the themes we offer: http://en.support.wordpress.com/custom-design/