How to theme the user profile page?
Here is a brief list of template suggestions for Drupal 7: http://drupal.org/node/1089656

If you are looking to theme the user page, there are multiple templates located in modules/user that can be overridden. The template you should probably start with is user-profile.tpl.php.  Copy that file and paste it in your theme directory then edit it as you like and clear the cache and you should see the changes.