Hello
Please name the file as index.html or index.htm.That will automatically open that page when you access the url www.mywebsite.com/directory
If you want to use a diffrent page create a .htaccess file inside the directory and To change the page that will be displayed when a visitor access a directory, write:
DirectoryIndex <necessary page>
It is possible to specify several pages:
DirectoryIndex index.shtml index.php index.php3 index.html index.htm