In order set the default documents or "index" documents for your Linux-hosted web site, create an .htaccess file and upload it to the root folder for your domain (see Web Hosting: Upload Files to Web Hosting Account), or add the following to an existing .htaccess file:
DirectoryIndex index.html index.htm index.php
You can modify the above example to include the name(s) of the default document(s) you would like to use.