Our servers are configured to use the following file names for start/main pages:
index.html
index.htm
index.shtml
index.php

The order above also shows the index files priority.
Let's say for example you have index.html, index.php and index.htm in your account's main folder.
The server will take as start page the index.html file, as it has highest priority.

You can change the default/main page priority or add other files to the list above by using .htaccess file. You need to create an .htaccess file in your site's main folder and place the following line in it:
DirectoryIndex filename.htmlWhere filename.html will be the name of your default/startup page. You can define several pages by placing the names separated by spaces, for example:
DirectoryIndex filename.html otherfile.php mypage.htm