Themes cannot be uploaded to WordPress.com. WordPress.com is a hosted service and you do not have FTP access. That's a feature of self-hosted blogs through WordPress.org. You can learn more about this in our support documentation. We do offer over 100 gorgeous themes which you can browse in our Themes Showcase.
If you require a custom theme, you might be interested in a self-hosted (WordPress.org) installation. You can learn more about the differences here.
We have hosting recommendations at the following link if you are interested: http://get.wp.com/hosting/
We also offer a Guided Transfer upgrade - we'll move everything over for you and get you all setup with your self-hosted WordPress.org installation.