What filenames should I use for my webpages?
Filename extensions DO matter. An HTML file should be called NAME.htm or NAME.html or even NAME.php. If you don't use one of these known extensions, the webserver will still serve the file but it may have a wrong MIME type and the browser won't know what to do with it.
Where should I put these files on the webserver?
In the public_html folder. If you make a file called contact.htm and put it in public_html, then its web address will be http://www.EXAMPLE.COM/contact.htm
If the site has many pages, it helps to use folders for different topics: e.g. files you put into public_html/support folder will appear in EXAMPLE.COM/support
What should I name the default page?
The “default page” for each folder is the one the webserver will serve when a filename isn't supplied by the visitor. E.g. they type in EXAMPLE.COM without anything after the .COM. The default page for each folder is usually named index.htm, index.html or index.php. This can be changed in the webserver configuration if necessary.
Making simple URLs
Let's say you want to make a page at EXAMPLE.COM/support - easy to type and remember.