Never alter a default theme like Twenty Eleven directly. Use a Child Theme. A minimal child theme just includes a single file called style.css. This file simply includes the mandatory comments and an import command to import the parent theme's style sheet.
You may have to redo some of your theme and menu configurations when you activate your child theme.
All the above takes about 10 minutes.
Since you appear to be using a Web Host who restricts your access, you will need to create your new files offline and upload them using your FTP account. If that doesn't work, you'll need to talk to GoDaddy.
When all that's in place, simply add your new style rules to your style.css file (offline and uploading if you need to).
Somebody else who hosts with GoDaddy may be able to give you more specific instructions. From what I've read, GoDaddy sounds like a really poor choice for a host.
HTH
Cheers
PAE