Set link location on logo only
The answer to your question very much depends on your how your theme is built and what you want to accomplish. If you want just to link the site logo to another location, I would suggest you to find a corresponding piece of code in page.tpl.php of your theme and replace it with correct link.

If you want to give your site administrators  ability to change that link to whatever they want, then I would do this this way:


Create a module that adds a page to admin menu of drupal (using hook_menu).
On that page render a form with one input field. When the form is loaded extract previously saved value using variable_get function.
On form submit save the form value using variable_set function
In your hook_preprocess_page extract value using variable_get, compose correct link and pass it to the template
In your page.tpl.php just render it in the proper place