web servers have to know which is the first page of your website. They do this by having a list of files to looks for 99% of the servers will want to find either an index.htm(l) file or a default.htm(l) one. Your hosting service will tell you which one to use.
More advanced programmers might be able to use index.cgi, index.php or index.asp. Remember if you are using your own desktop server to see which one to use refer to the config file or the setup program.