If you have multiple htmls in a folder, how do you make one of the html show up, when you type in the folder name.

Ex:
www.domain.com
has a folder called seo
In seo folder:
analyzer.html, meta.html, keyword.html, blabla.html
Now when i type in www.domain/seo/
How do i make one of these html be the main html of the directory folder name that is typed?
Thank you!