There are a number of ways to go about that. For one, I'd recommend uploading a logo that is an appropriate size for what you're trying to accomplish. But like any Drupal site, you can do as much or as little as you want to the design to make it function. To get you started on learning about theming for Drupal, check out this resource: http://drupal.org/documentation/theme or look into other themes that may be better for you: http://www.drupalcommerce.org/extensions/theme