You don’t need one. When you look at your site via ftp, you will see a folder that has the same name as your domain name. All files inside that folder are visible from the web.
If you want to you can create new folders inside of that folder but they are not required.
Therefore if your domain was called example.com and you placed a web page inside of the folder called example.com called nextpage.html, your url would be http://www.example.com/nextpage.html
Now if you do not want someone to have to type in the page for example: http://www.example.com
Just create a webpage that starts with the name index and use one of the allowed extensions such as htm, html, php and so on.
If you create a folder called folder and used the index file. then the url would be http://www.example.com/folder or they could type http://www.example.com/folder/index.html if your index uses the html extension.
I hope this all makes sense. Any more questions feel free to ask. There are a lot of very friendly people that are willing to help you.