Sonic.net

Hello Sonic friends. I want to customize my WordPress installation by creating a child theme before I change more of the existing code in the Twenty Eleven theme.
The online info I find tells me to create a new file in the wp-content/themes directory. On my site (unofficialbookreviewer.com) that extension just takes me to a blank page.
Where is the directory and how do I create a new file in it?
Thanks for any help you can offer.

I'm happy to report that I found a solution: a plugin that allows you to create child themes at shared hosts like Sonic. If anybody else is interested, here's the link: http://wordpress.org/extend/plugins/one ... ild-theme/